15^ 

w*"-.~-"*BKMi 


THE  ROMAN  COMAGMATIC 

REGION 


BY 


HENRY  S.  WASHINGTON 


WASHINGTON,  D.  C. 

PUBLISHED  BY  THE    CARNEGIE  INSTITUTION    OF   WASHINGTON 
1906 


HE  ROMAN  COMAGMATIC 
REGION 


BY 


HENRY  S.  WASHINGTON 


WASHINGTON,  D.  C. 

PUBLISHED  BY  THE  CARNEGIE  INSTITUTION  OF  WASHINGTON 
1906 


CARNEGIE  INSTITUTION    OF    WASHINGTON 
PUBLICATION  No.  57 


PRINTED   AT   THE    UNIVERSITY  OF   CHICAGO   PRESS,   CHICAGO 


CONTENTS. 

PACK 

Introduction v 

General  topography  and  geology i 

Vulsinian  District 3 

Ciminian  District 5 

Sabatinian  District 5 

Latian  District 6 

Hernican  District 7 

Auruncan  District 7 

Campanian   District 8 

Petrography 9 

Introduction 9 

Chemical   analyses 10 

Classification n 

Descriptions  and  names  of  the  rocks 12 

The  formal  descriptions 15 

Transitional   types 17 

Determination  of  minor  constituents 18 

Ischial  nordmarkose-phlegrose  [augite-trachyte] 19 

Cumal  phlegrose    [phonolitic  trachyte] 22 

Rotaral  phlegrose  [trachyte  obsidian] 27 

Bolsenal  vulsinose  [vulsinite] 30 

Viterbal  vulsinose  [leucite-trachyte] 34 

Pallanzanal  vulsinose  [leucite-trachyte] 40 

Paglial  procenose-pulaskose  [leucite-trachyte] 41 

Sabatinal  beemerose  [leucite-phonolite] 46 

Tavolatal  janeirose-appianose  [leucite-tephrite,  tavolatite] 50 

Sorianal  harzose  [biotite-latite] 54 

Arsal  vulsinose-ciminose  [vulsinite] 58 

Fiescolal  ciminose  [ciminite] 62 

Bagnoreal  ciminose  [leucite-trachyte] 67 

Martinal  vicose-ciminose  [leucite-tephrite] 71 

Arsal  monzonose  [vulsinife] 74 

Foglianal  ciminose-auruncose  [leucite-tephrite] 79 

Teanal  ciminose-auruncose  [leucite-trachyte] 82 

Orvietal  auruncose  [leucite-tephrite] 85 

Monfinal  shoshonose  [biotite-latite] 88 

Foglianal  vicose  [leucite-tephrite] 91 

Bagnoreal  vicose  [leucite-tephrite] 96 

Orvietal  vicose  [leucite-tephrite] 100 

Vesbal  braccianose  [leucite-tephrite] 103 

Sommal  braccianose  [leucite-tephrite] 107 

Galeral  braccianose  [leucitite] 108 

Hernical  braccianose  [leucitite] in 

Atrial  braccianose  [leucite-tephrite] 115 

Scalal  vesuvose-braccianose  [leucite-tephrite] 117 

in 


IV  CONTENTS. 

PAGE 

Romal  vesuvose  [leucitite] 121 

Galeral  albanose-jugose  [leucitite] 123 

Fiordinal  fiasconose  [leucite-basanite] 126 

Romal  albanose  [leucitite] 130 

Saccal  albanose  [leucitite] 133 

Boval  albanose  [melilitic  leucitite,  cecilite] 138 

Correlation  of  types 141 

Petrology 144 

Geologic  occurrence 144 

Chemical  characters 145 

Normative  characters 151 

Relations  of  norm  and  mode 154 

Mineralogical  characters 155 

Soda-orthoclase 156 

Soda-lime  feldspar 156 

Leucite 157 

Nephelite 157 

Sodalite  and  haiiyne 157 

Augite 157 

Hypersthene       . 158 

Biotite 158 

Olivine 158 

Melilite 159 

Absence  of  minerals 159 

Textural   characters 159 

Space  relations 160 

Vulsinian  District 161 

Ciminian  District 162 

Sabatinian  District 163 

Latian   District 164 

Hernican  District 164 

Auruncan  District 164 

Campanian   District 165 

Distribution  of  magmas 166 

Progression  of  types 169 

The  rocks  of  the  Tuscan  Region 170 

Quantitative  relations        170 

Quantitative  relations  of  the  magmas 172 

The  average  magma 173 

Time  relations 175 

Geologic  age  of  the  eruptions 175 

Order  of  succession  of  the  types 176 

Comparison  with  other  regions 177 

The  formation  of  leucite 181 

The  Roman  Region 181 

Conclusions 185 

The  distribution  of  barium 188 

Bibliography 192 

Index 195 


INTRODUCTION. 

The  present  paper  embodies  the  results  of  an  investigation  undertaken  under 
the  auspices  of  the  Carnegie  Institution  of  Washington,  and  is  the  continuation 
and  completion  of  some  studies  which  were  published  several  years  ago.  Since  the 
first  publications  much  additional  material  has  been  collected,  and,  while  my  collec- 
tions do  not  pretend  to  be  exhaustive  or  complete,  the  specimens  are  so  numerous 
and  are  regarded  as  so  representative  of  the  rocks  of  the  region  that  they  may  prop- 
erly serve  as  a  basis  for  some  general  discussion. 

One  of  the  objects  of  the  paper  is  the  detailed  description  of  the  many  rare  rock 
types  characterized  by  the  presence  of  leucite,  for  which  the  Italian  volcanoes  are 
so  famous,  and  most  of  which  are  very  inadequately  known.  It  was  thought  also 
that  a  careful  chemical  study  of  these,  as  well  as  of  the  accompanying  non-leucitic 
rocks,  might  throw  some  light  on  the  magmatic  and  physical  conditions  which  are 
involved  in  and  control  the  formation  of  this  rare  and  interesting  mineral. 

Another  aim  was  the  description  and  discussion  of  the  characters  of  the  region 
as  a  whole,  as  an  addition  to  the  small  but  growing  list  of  petrographic  provinces 
which  are  more  or  less  adequately  known  in  their  petrological  and  petrographical 
aspects.  The  study  of  these  would  seem  to  have  important  bearings  on  some  of 
the  broader  problems  of  petrology,  especially  the  theory  of  differentiation  and  the 
question  of  the  original  homogeneity  or  heterogeneity  of  the  earth.  As  regards 
these  we  are  still  in  the  observing  and  fact-collecting  stage  of  the  science,  and  it 
was  thought  that  the  description  of  the  region  from  the  petrological,  as  well  as  from 
the  petrographical,  point  of  view  would  be  of  some  value. 

In  this  connection  attention  may  be  called  to  the  introduction  of  the  term  "  co- 
magmatic  region "  to  replace  the  older  "petrographical  province."  It  is  thought 
that  the  latter  is  unsatisfactory  in  that  it  implies  only  the  purely  petrographical 
characters,  without  reference  to  the  broader  petrological  characters  and  relations  (of 
which  the  petrographical  are  but  one  set),  the  knowledge  of  which  is  essential  to  our 
understanding  of  a  given  area  of  genetically  related  rocks  as  a  whole.  Further- 
more, the  term  "province"  implies  that  the  area  is  part  of  a  larger  one,  and,  while 
this  may  ultimately  be  found  to  be  true  in  some  instances,  it  would  seem  better  to 
employ  some  term  which  does  not  have  this  connotation  and  which  does  not  postu- 
late in  the  term  itself  any  relations  to  others. 

It  may  be  objected  to  the  word  "comagmatic"  that  it  begs  the  question  of 
common  genetic  origin  for  the  rocks  of  a  given  region.  But  the  word  is  used  merely 
with  the  idea  that  the  magmas  of  a  certain  area  have  characteristics  in  common, 
which  is  the  idea  underlying  the  use  of  "petrographical  province,"  and  whether  the 
occurrence  of  these  common  characters  is  due  to  processes  of  magmatic  differentia- 
tion or  to  other  causes  is  not  asserted  or  implied  in  the  term. 

In  the  case  of  the  present  region,  as  in  many  others,  it  will  be  found  that  it  is 
naturally  divisible  into  smaller  groups,  around  clustered  or  single  centers  of  intru- 

V 


VI  INTRODUCTION. 

sion  or  eruption.  For  these  the  term  "district"  is  used,  the  individual  points  from 
which  the  igneous  rocks  were  extruded  being  called  "centers." 

Advantage  was  also  taken  of  this  opportunity  to  illustrate  some  of  the  prac- 
tical applications  to  the  study  of  rocks  of  the  principles  of  the  new  classification  and 
nomenclature,  and  to  test  their  value  in  petrological  investigations.  Such*  an  expo- 
sition was  especially  needed  in  the  matter  of  types  and  the  use  of  the  proposed  rock 
names.  In  recent  literature  a  number  of  petrographers  have  employed  the  classi- 
fication so  far  as  to  give  the  magmatic  position  of  the  various  rocks  dealt  with.  But 
the  system,  to  be  a  practical  one,  as  it  is  intended  to  be,  must  go  farther  than  this, 
and  its  rock  names  and  other  parts  of  its  nomenclature  must  be  capable  of  use  in 
descriptive  literature,  just  as  are  those  of  the  prevailing  systems. 

No  apology  is  therefore  offered  for  the  language  in  which  part  of  the  paper  is 
couched,  nor  for  the  unfamiliar  terms  employed  throughout.  On  the  contrary,  it 
is  hoped  that  their  use  here  will  aid  petrographers  to  a  better  understanding  of  the 
quantitative  system,  and  that  the  paper  will  serve  in  some  sort  as  a  working  model 
of  its  methods  and  applications.  Some  of  the  terms  which  are  not  to  be  found  in 
the  original  publication  of  the  quantitative  system  are  the  result  of  subsequent  con- 
ferences and  discussions  between  the  joint  authors  of  that,  and  they,  as  well  as  fur- 
ther additions,  are  expected  to  appear  in  the  Journal  oj  Geology  (vol.  xiv,  1906). 

The  whole  region  whose  rocks  are  the  present  objects  of  study  is  being  investi- 
gated with  enthusiasm,  especially  from  the  geological  and  structural  points  of  view,  by 
many  able  Italian  geologists,  whose  published  results  have  been  made  use  of  largely. 
Some  of  the  more  important  of  these  publications,  with  those  of  foreign  observers, 
will  be  found  in  the  bibliographical  list  given  later.  With  comparatively  limited 
opportunities  to  make  the  many  detailed  observations  in  the  field  which  are  needed 
to  elucidate  all  the  strictly  geological  questions  involved  in  the  study  of  the  region, 
I  can  not  venture  to  treat  these  adequately  on  the  basis  of  my  own  knowledge  and 
must  leave  their  discussion  to  the  geologists  of  the  country,  who  have  shown  such 
zeal  in  the  study  of  the  many  interesting  problems  which  their  favored  peninsula 
presents.  I  can  only  express  the  hope  that  my  Italian  confreres  will  not  deem  me 
an  unwelcome  intruder  into  their  proper  sphere  of  investigation,  but  will  consider 
the  efforts  made  here  to  solve  some  of  the  special  problems  as  evidence  of  the  deep 
interest  taken  in  the  geology  of  their  country,  whose  men  of  science  have  always 
shown  themselves  most  hospitable  to  those  from  abroad. 

It  is  a  great  pleasure  to  express  my  thanks  to  the  Trustees  of  the  Carnegie 
Institution  of  Washington,  who  have  so  generously  aided  me  in  these  investiga- 
tions, and  have  made  it  possible  to  bring  to  completion  a  long-cherished  plan.  To 
my  colleagues  in  the  quantitative  classification,  Doctor  Cross,  Professor  Iddings,  and 
Professor  Pirsson,  I  am  deeply  indebted  for  much  kind  interest,  advice,  and  aid  in 
many  ways.  As  one  of  the  objects  of  this  paper  is  to  serve  as  an  example  of  the 
practical  application  of  our  ideas,  they  have  been  frequently  consulted,  part  of  the 
manuscript  has  been  examined  and  commented  on  by  them,  and  the  general  form 
in  which  the  descriptions  of  the  types  are  cast  is  the  result  of  our  joint  discussions. 


THE     ROMAN    COMAGMATIC    REGION. 


GENERAL  TOPOGRAPHY  AND  GEOLOGY. 
Introduction. 

The  volcanoes  of  the  Italian  peninsula  may  be  referred  to  several  distinct  co- 
magmatic  regions  (petrographic  provinces),  the  distinctions  between  them  being 
based  largely  upon  the  petrographic  and  chemical  characters  of  their  rocks,  though 
they  are  also  separated  by  their  geologic  ages  and  their  topographic  and  geologic 
relations.  The  most  prominent  of  these  may  be  enumerated  as  follows,  and  are 
shown  in  the  map  on  the  following  page: 

The  Venetian  Region,  comprising  the  volcanic  complexes  of  the  Euganean  and 
Berican  Hills. 

The  Apulian  Region,  comprising  the  volcanic  complex  of  Monte  Vulture. 

The  Tuscan  Region,  comprising  the  volcanoes  of  Monte  Amiata,  Montecatini, 
Orciatico,  Campiglia  and  Roccastrada,  with  which  may  be  placed  the  volcanoes 
of  Tolfa  and  Cerveteri,  and  Monti  Calvario  and  San  Vito  near  Bracciano. 

The  Roman  Region,  comprising  the  main  line  of  volcanoes  which  extend  from 
Lake  Bolsena  southeast  to  Vesuvius  and  the  Phlegrean  Fields. 

All  of  these  vary  widely  in  the  characters  of  their  eruptive  rocks,  and  differ  as 
well  in  regard  to  their  geological  ages.  The  only  one  which  concerns  us  here  is  the 
last  mentioned,  the  Roman  Region,  so  named  from  the  fact  that  it  embraces  the  chief 
territory  of  the  ancient  Roman  Republic,  as  well  as  the  capital  city  of  modern  Italy. 
These  volcanoes  are  all  of  Quaternary  age,  the  eruptions  of  some  of  them  extending 
into  historic  and  even  modern  times.  Extending  in  a  narrow  line  between  the  Apen- 
nines and  the  Tyrrhenian  Sea,  these  volcanoes  occupy  the  sites  of  old  embayments 
in  the  Mesozoic  and  Tertiary  sediments  which  form  the  backbone  of  the  Italian 
peninsula. 

It  will  be  found  convenient  to  take  up  the  description  of  the  region  under  the 
different  districts  into  which  it  may  be  subdivided,  which  surround  distinct  centers 
of  volcanic  activity,  and  which  are  more  or  less  clearly  separated  from  one  another. 
These  districts  are  seven  in  number,  and  may  be  thus  enumerated,  the  names  be- 
stowed upon  them  following  as  far  as  possible  the  usages  of  the  Italian  geologists: 

(1)  Vulsinian  District,  comprising  the  volcanic  complex  around  Lake  Bolsena. 
Named  from  the  ancient  tribe  of  the  Vulsinii. 

(2)  Ciminian  District,  comprising  the  volcanoes  near  Viterbo.     Named  from 
the  Latin  appellation  of  Colli  Ciminii. 

(3)  Sabatinian  District,  comprising  the  volcanic  complex  around  Lake  Brac- 
ciano.    Named  from  the  Roman  name  of  the  lake,  Lacus  Sabatinus. 


THE  ROMAN  COMAGMATIC  REGION. 


FIG.  i. — Sketch  map  of  Italian  comagmatic  regions. 


Roman  Region. 
Tuscan  Region. 
Venetian  Region. 
Apulian  Region. 

V  =Vulsinian  District. 
Ci  =  Ciminian  District. 

S  =  Sabatinian  District. 
L  "=Latian  District. 

H  =Hernican  District. 
A  =Auruncan  District. 

Ca=Campanian  District. 

Ve=  Vesbian  Volcano. 
P  =Phlegrean  Fields. 

I   =Ischia. 
i    «=Montecatini. 

2    =  Campiglia. 
3   =Massa  Marittima. 

4   =Roccastrada. 

S    •=  Monte  Amiata. 
6   =Tolfa. 

7    =  Cerveteri. 
B  =Berican  Hills. 

E  =Euganean  Hills. 
Vu=  Monte  Vulture. 

Et  =Etna. 

GENERAL  TOPOGRAPHY  AND  GEOLOGY.  3 

(4)  Latian  District,  comprising  the  volcanic  complex  of  the  Alban  Hills  near 
Rome.     It  is  so  called  from  its  position  in  the  ancient  Latium. 

(5)  Hernican  District,  comprising  the  volcanoes  in  the  valley  of  the  Sacco 
River,  formerly  inhabited  by  the  Hernici,  whence  the  name  is  derived. 

(6)  Auruncan  District,  comprising  the  volcano  of  Rocca  Monfina.    The  name 
is  derived  from  the  ancient  tribe  of  the  Aurunci. 

(7)  Campanian  District,  comprising  the  volcanoes  of  Vesuvius,  the  Phlegrean 
Fields,  and  of  Ischia.    It  is  named  from  the  Roman  province  of   Campania,  in 
which  it  lies. 

In  addition  there  are  a  number  of  very  small  cones,  or  remains  of  cones,  at  some 
distance  from  the  main  line,  as  at  Radicofani,  San  Venanzo,  and  Rieti.  The  rocks 
of  these  are  possibly  connected  with  those  of  the  main  line,  but  they  will  not  be 
described,  and  their  rocks  will  be  only  incidentally  mentioned. 

The  relations  of  the  volcanoes  of  the  Roman  Region  to  the  earlier  ones  of  the 
Tuscan  Region,  which  lie  rather  closely  to  the  north  and  west,  are  problematical. 
There  is  some  evidence  that  the  rocks  of  the  two  regions  are  connected  genetically, 
as  will  be  pointed  out  later,  but  it  seems  advisable  for  the  present  to  confine  our 
attention  to  the  Roman  Region,  leaving  the  discussion  of  the  volcanoes  and  rocks 
of  the  Tuscan  Region  to  the  future. 

The  brief  descriptions  which  follow  do  not  pretend  to  be  complete.  They 
serve  only  to  give  a  general  idea  of  the  topography  and  geology  and  the  vulcano- 
logical  structure  of  the  several  districts,  sufficient  for  a  proper  understanding  of  some 
of  the  petrological  relations  to  be  discussed  in  subsequent  pages.  They  are  based  on 
the  most  modern  literature  available,  which  is  due  to  the  activity  of  the  able  Italian 
and  German  geologists  who  have  studied  the  region,  supplemented  by  the  observa- 
tions which  I  was  able  to  make. 

Vulsinian  District. 

This  district,  the  most  northerly  of  the  zone,  extends  from  the  Paglia  River  on 
the  north  to  the  Ciminian  District  on  the  south,  the  boundary  between  the  two 
being  irregular  and  in  places  not  well  denned.  It  embraces  an  area  of  about  2,280 
square  kilometers,  and  extends  in  all  directions  around  Lake  Bolsena,  which  is 
the  most  prominent  topographical  feature. 

The  lake  is  quite  regularly  elliptical  in  shape,  the  major  axis  running  north  and 
south.  Its  dimensions  are  13  by  n  kilometers,  with  an  area  of  114.5  square  kilo- 
meters. The  lake  surface  lies  305  meters  above  sea-level,  and  the  bottom,  with 
some  irregularities,  slopes  in  general  toward  a  point  near  the  center,  where  the  depth 
is  the  maximum  one  of  146  meters.  Two  small  islands  project  above  the  water  in 
the  southern  half. 

Lake  Bolsena  is  surrounded  by  a  girdle  of  hills  which  slope  steeply  down 
to  the  narrow  fringing  shore.  These  are  the  highest  on  the  north,  where  they 
attain  an  altitude  of  702  meters  above  sea-level,  and  among  these  are  the  small 


4  THE  ROMAN  COMAGMATIC  REGION. 

towns  of  San  Lorenzo  and  Acquapendente.  On  the  east  the  hills  are  of  a  fairly 
uniform  height  of  600  to  650  meters,  the  town  of  Bolsena  lying  on  the  northeastern 
shore,  and  the  city  of  Orvieto  some  14  kilometers  northeast  of  the  lake,  on  the  edge 
of  the  igneous  area.  The  town  of  Montefiascone  is  situated  on  a  hill  (633  meters) 
near  the  southeastern  corner  of  the  lake.  On  the  south  the  hills  are  low,  uniformly 
less  than  100  meters  above  the  lake-level.  On  this  shore  are  the  villages  of  Marta 
(at  the  lake  emissary)  and  Capodimonte,  while  15  kilometers  to  the  south  is  the 
town  of  Toscanella.  The  western  hills  are  also  low,  though  somewhat  higher  than 
on  the  south,  with  the  town  of  Valentano  near  the  southwestern  corner. 

From  this  encircling  crest  of  hills  the  land  surface  slopes  down  to  the  north, 
east,  and  west,  the  fall  being  quite  regular,  but  somewhat  diversified  by  the  remains 
of  cones  and  craters.  As  the  rocks  are  to  a  large  extent  somewhat  incoherent  tuffs, 
the  general  topography  is  characterized  by  erosion  ravines,  radiating  from  the  lake, 
and  often  leaving  isolated  buttes  here  and  there. 

To  the  west  of  Lake  Bolsena  is  another,  though  much  smaller,  depression,  or 
rather  plain,  surrounded  by  a  girdle  of  hills,  whose  circle  impinges  on  that  of 
Lake  Bolsena.  In  this  is  the  small  Lake  Mezzano,  and  the  whole,  undoubtedly  a 
distinct  crater,  is  known  as  the  Latera  Volcano,  from  a  small  town  on  its  rim.  A 
prominent  feature  of  this  is  an  extensive  and  very  rough-surfaced  lava  flow,  now 
covered  with  forest,  and  known  as  the  Selva  del  Lamone.  Owing  to  bad  weather 
I  was  unfortunately  unable  to  visit  the  Latera  Volcano,  though  my  examination  of 
the  rest  of  the  district  was  fairly  complete. 

The  general  structure  of  the  complex  (and  especially  the  origin  of  Lake 
Bolsena)  has  long  been  the  subject  of  dispute,  and  unfortunately  the  two  geologists 
who  have  studied  the  district  most  completely  (Moderni  and  Sabatini)  do  not 
hold  the  same  views.  According  to  Moderni,  the  Vulsinian  complex  has  been 
formed  by  the  eruptions  from  four  principal  vents — the  volcanoes  of  Latera,  Bolsena, 
Montefiascone,  and  Capodimonte — the  first  three  being  situated,  respectively,  on  the 
west,  northeast,  and  southeast,  and  the  last  being  to  a  large  extent  now  covered  by 
the  waters  of  the  lake  in  its  southern  part.  Each  of  these  volcanoes  had  its  own 
smaller  flanking  and  parasitic  cones.  Lake  Bolsena,  according  to  his  view,  is  not 
a  true  crater  lake,  due  to  either  the  explosion  or  the  falling  in  of  a  single  large  vol- 
canic cone,  but  rather  a  lake  basin  formed  by  the  accumulation  of  the  materials 
ejected  from  the  four  surrounding  volcanoes. 

On  the  other  hand,  Sabatini  regards  the  complex  as  essentially  a  system  of 
concentric  crater  ridges  and  atrios,  due  to  the  gradually  decreasing  vulcanicity 
about  one  main  central  vent,  of  which  Lake  Bolsena  is  the  crater.  Superposed  on 
the  flanks  of,  and  later  than,  this  main  volcano  are  the  smaller  ones  of  Latera,  Bol- 
sena, and  Montefiascone,  with  their  parasitic  cones. 

It  is  unnecessary  here  to  weigh  these  two  opinions,  so  diametrically  opposed  in 
some  respects.  Each  represents  the  conclusions  of  an  able  geologist  who  has  made 
a  careful  and  profound  study  of  the  district,  Moderni  more  especially  of  the  western 


GENERAL  TOPOGRAPHY  AND  GEOLOGY.  5 

half,  and  Sabatini  of  the  eastern.  For  the  present,  the  question  must  be  left  an  open 
one,  and  this  is  the  more  advisable  here,  since,  whatever  be  the  facts  as  to  the  origin 
of  the  topographic  features  and  structure,  they  have  little  bearing  on  the  predom- 
inantly petrological  questions  treated  of  in  the  present  paper. 

Ciminian  District. 

The  Ciminian  District  immediately  adjoins  the  preceding  one  on  the  north,  and 
is  bounded  on  the  south  by  the  Sabatinian  District.  Its  area  is  approximately  900 
square  kilometers.  It  is  composed  of  two  distinct  volcanoes,  that  of  Cimino  on  the 
north  and  of  Vico  on  the  south,  the  former  being  the  earlier. 

The  remains  of  the  Cimino  Volcano  are  made  up  of  several  hills,  including 
Monti  Pallanzana,  Vitorchiano,  Cigliano,  and  Soriano,  surrounding  on  the  west, 
north,  and  east  the  main  mass  of  Monte  Cimino  proper,  whose  highest  point  has  an 
altitude  of  1,035  meters  above  sea-level.  This  complex  is  pretty  clearly  the  remnant 
of  a  single  volcano,  though  the  original  structure  is  largely  hidden  by  the  extensive 
erosion  which  it  has  undergone  and  which  has  been  much  aided  by  the  tuff-like 
character  of  most  of  its  rocks. 

Immediately  to  the  south,  or  rather  south-southwest,  of  Monte  Cimino,  whose 
southern  flanks  are  partly  covered  by  its  ejectamenta,  is  the  better-preserved  Vico  Vol- 
cano. Vico  is  a  fairly  well-preserved  volcanic  cone  of  the  normal  type  of  strato- volcano, 
the  sides  sloping  up  regularly  (much  more  so  than  in  the  Vulsinian  District)  to  the 
circular  crest,  inside  which  is  the  small,  shallow  Lake  Vico.  North  of  this  lake  is 
the  site  of  the  last  eruption  of  the  volcano,  Monte  Venere,  a  small  hill,  rising  325 
meters  above  the  lake-level,  and  composed  of  lava  flows  with  some  tuffs  and  lapilli. 
The  lake  surface  is  507  meters  above  sea-level,  and  the  steep  inner  wall  of  Vico  rises 
from  100  meters  above  this  on  the  south,  where  it  is  the  lowest,  to  456  meters  at 
Monte  Fogliano  on  the  west.  Though  the  almost  perfect  circularity  of  the  rim  is 
marred  by  the  projection  of  this  last  summit,  there  seems  to  be  no  doubt  that  the 
hollow  is  a  true  crater  and  the  volcano  a  simple  one,  with  few  parasitic  cones.  The 
feature  of  radial  valleys  eroded  in  the  loose  surrounding  tuffs  is  well  marked  in  the 
topography  of  this  volcano. 

Of  the  towns  which  may  be  mentioned,  the  city  of  Viterbo  lies  in  the  angle 
between  the  two  volcanoes,  west  of  Soriano  and  north  of  Vico.  About  the  Cimino 
Volcano  we  find  the  villages  of  Soriano  on  the  east,  Vitorchiano  a  short  distance  to 
the  north,  and  Bagnaia  on  the  northwest,  while  around  Vico  are  San  Martino  near 
the  northwest  crater  rim.  Vetralla  on  the  west,  Capranica  on  the  southern  edge  of 
the  area,  Ronciglione  near  the  southeast  shore  of  Lake  Vico,  and  Caprarola  and 
Canepina  to  the  east  and  northeast. 

Sabatinian  District. 

This  lies  immediately  to  the  south  of  the  Vico  Volcano,  with  whose  tuffs  its  own 
intermingle.  The  area  covered  by  volcanic  rocks  is  estimated  by  Moderni  at  1,369 
square  kilometers,  so  that  in  extent  it  is  intermediate  between  the  Vulsinian  and  the 


6  THE  ROMAN  COMAGMATIC  REGION. 

Ciminian  districts.  It  resembles  the  district  first  described,  inasmuch  as  a  sheet  of 
water,  Lake  Bracciano,  forms  the  topographic  center.  This  lake,  with  an  area  of 
57.5  kilometers  and  a  depth  of  160  meters,  has  its  surface  164  meters  above  sea- 
level.  It  is  approximately  circular  in  shape,  with  two  or  three  small  bays,  and 
is  surrounded  on  the  west  and  north  by  a  girdle  of  hills,  which  attain  their  highest 
altitude  above  sea-level  (602  meters)  at  Monte  di  Rocca  Romana  on  the  north.  Im- 
mediately to  the  east  is  the  small  Lago  di  Martignano,  and  the  basins  of  two  others 
are  represented  in  the  Stracciacappa  Marsh  and  the  Valle  di  Baccano,  all  three 
being  distinctly  elliptical  or  circular  and  surrounded  by  rings  of  hills  or  curving 
ridges.  From  the  inner  wall  surrounding  Lake  Bracciano  the  land  slopes  down  to 
the  west,  north  and  east  rather  regularly,  though  it  is  broken  on  the  west  by  the 
Tertiary  volcanic  masses  of  Monte  Calvario  and  Monte  San  Vito.  On  the  south  the 
hills  along  the  lake  shore  are  lower,  and  the  slope  southward  is  very  gentle. 

The  general  topographic  resemblance  to  the  Vulsinian  District  is  very  marked, 
though  the  features  are  on  a  somewhat  smaller  scale.  Moderni,  who  has  studied 
this  district  very  carefully,  argues  for  it  a  structure  analogous  to  that  which  he 
assigns  to  the  Vulsinian.  He  regards  Lake  Bracciano  as  being,  not  a  crater  lake, 
but  as  a  lake  basin  formed  by  the  ejections  of  the  surrounding  volcanic  vents,  of 
which  he  enumerates  and  describes  eight  main  ones,  with  their  parasitic  cones.  This 
view  is  combated  by  Sabatini,  so  that,  for  the  present,  the  question  of  the  structure 
of  the  complex  and  the  origin  of  the  lake  must  be  regarded  as  unsettled. 

Of  the  towns  in  the  district  the  most  important  is  Bracciano,  on  the  crest  of 
the  western  late-rim.  Smaller  ones  are  Manziana  and  Oriolo  on  the  northwest, 
Trevignano  on  the  northern  shore,  near  a  small  semicircular  bay,  and  Anguillara 
on  the  southeast  shore  near  the  outlet  of  the  lake. 

Latian  District. 

The  Alban  Hills,  which  make  up  this  district,  are  situated  to  the  southeast  of 
Rome,  as  is  well  known,  and  some  50  kilometers  southeast  of  Lake  Bracciano.  The 
area  covered  by  the  volcanic  complex  is  about  580  square  kilometers. 

The  complex  consists  of  a  circle  of  hills  and  summits,  varying  in  altitude  from 
550  to  939  meters,  and  with  a  diameter  of  about  15  kilometers.  Up  to  this  the  sur- 
face rises  gently  from  the  north,  east,  and  south,  while  on  the  west  and  southwest 
the  regularity  of  this  somma  is  broken  by  the  lakes  Albano  and  Nemi  and  the  Valle 
Ariccia,  the  sites  of  late  flanking  eruptions.  Inside  the  large  circle  there  rises  a 
fairly  well-defined  cone,  attaining  a  height  of  956  meters  and  with  the  crater  known 
as  Campo  d'Annibale  at  the  top.  Monte  Cavo  forms  the  most  prominent  and 
almost  the  highest  point  of  the  rim  encircling  this. 

The  structure  is  explained  by  Sabatini,  who  has  made  a  careful  study  of  the 
complex,  as  analogous  to  that  of  Somma  and  Vesuvius.  The  larger  circle  of  hills 
is  the  crater  of  the  earliest  volcano,  within  which,  at  a  later  date,  was  thrown  up  the 
interior  cone  of  Monte  Cavo.  The  small  lakes  occupying  the  western  slope,  and 


GENERAL  TOPOGRAPHY  AND  GEOLOGY.  7 

which  destroyed  much  of  the  earliest  crater  wall,  are  true  crater  lakes,  due  to  later 
eruptions,  the  positions  of  the  vents  having  shifted.  The  tuffs  from  this  volcano 
have  covered  nearly  the  whole  of  ancient  Latium. 

The  most  important  towns  to  be  mentioned  are  Rocca  di  Papa  near  Monte 
Cavo,  Grotta  Ferrata,  and  Frascati  on  the  northwest  outer  slope  of  the  earliest 
cone,  Rocca  Priora  on  the  northeast,  and  Velletri  on  the  southeast,  while  Marino, 
Castel  Gandolfo,  Albano,  and  Genzano  are  all  in  the  immediate  vicinity  of  the 
small,  late  crater  lakes  on  the  west  and  southwest. 

Hernican  District. 

This  district,  which  lies  in  the  valley  of  the  Sacco,  about  50  kilometers  east- 
southeast  of  the  Alban  Hills,  differs  from  those  described  above  in  two  respects — 
the  geologic  structure  and  the  volume  of  the  eruptive  products.  Instead  of  forming 
a  single  volcano  or  volcanic  complex  of  large  dimensions,  the  eruptions  were  of  such 
small  amount  and  the  vents  so  widely  separated  that  they  formed  only  six  or  eight 
small  and  isolated  cones.  These  may  be  called  the  Cones  of  Ticchiena  (between 
Frosinone  and  Ferentino),  Pofi,  San  Francesco,  San  Marco,  Sant'  Arcangelo,  Callame 
(the  last  four  around  the  town  of  Ceccano),  Giuliano,  Patrica,  Morolo,  and  Selva 
dei  Muli.  Of  these  the  one  which  best  preserves  any  well-defined  vestiges  of  its 
original  form  is  that  of  Pofi.  This  village  lies  on  a  hill  295  meters  above  sea-level 
and  130  above  the  Sacco  Valley  bottom,  which  is  formed  for  the  most  part  of  tuffs 
and  lapilli,  with  a  few  small  lava  flows,  the  crater  having  disappeared.  The  other 
cones  have  suffered  so  extensively  from  atmospheric  degradation  that  they  are  now 
represented  by  areas  of  volcanic  tuffs,  with  small  exposures  of  lava  here  and  there, 
and  with  but  little  trace  of  the  original  form.  The  total  area  of  the  volcanic  prod- 
ucts is  very  small — less  than  50  square  kilometers  in  all. 

Auruncan  District. 

This  district  is  situated  about  70  kilometers  southeast  of  the  preceding  one 
and  about  65  north-northwest  of  Vesuvius,  and  its  volcanic  rocks  cover  an  area  of 
about  400  square  kilometers,  roughly  estimated.  It  consists  of  but  one  volcano, 
which  is  called  Rocca  Monfina.  This  is  of  the  simple  type  of  strato- volcanoes,  analo- 
gous to  that  of  the  Alban  Hills  and  the  Somma-Vesuvius  mass,  and  has  compara- 
tively few  parasitic  cones.  It  possesses  an  external  ring,  sloping  regularly  outward 
on  all  sides  and  eroded  into  radial  valleys.  The  crest  of  the  ring  reaches  its  maxi- 
mum height  of  926  meters  above  sea-level  on  the  southwest  side,  which  is  the  best 
preserved,  the  original  crater  walls  having  been  much  degraded,  or  being  at  least 
much  lower,  on  the  north  and  east.  In  the  center  of  the  plain  inclosed  by  the  large 
crater  ring,  which  measures  about  6  kilometers  in  diameter,  rises  the  dome  of  Monte 
Santa  Croce,  the  site  of  the  final  eruption  of  the  volcano,  with  the  smaller  Monte 
Lattani  adjoining  it  on  the  north. 

Of  the  towns  around  this  volcano,  the  only  ones  which  need  be  mentioned  are 
Rocca  Monfina,  at  the  east  foot  of  Monte  Santa  Croce;  Teano,  near  the  southeastern 


8  THE  ROMAN  COMAGMATIC  REGION. 

edge  of  the  district;  Sessa  Aurunca,  similarly  placed  on  the  southwest;  and  Conca, 
on  the  northern  outer  slope. 

Campanian  District. 

This  district  may  be  conveniently  divided  into  three  subdistricts — the  Vesbian,* 
which  consists  of  the  Vesbian  Volcano,  a  convenient  appellation  for  the  mass  of 
Mount  Vesuvius  and  the  encircling  Monte  Somma;  the  Phlegrean,  embracing  the 
volcanoes  of  the  Phlegrean  Fields;  and  the  Ischian,  consisting  of  the  island  of  Ischia. 

The  Vesbian  Volcano  is  so  well  known  and  has  been  described  so  often  that 
little  need  be  said  of  it  here.  It  consists  of  the  older  ring  of  Somma,  which  forms 
a  half-circle  open  to  the  south,  with  a  maximum  elevation  of  1,137  nieters.  Inside 
this,  and  slightly  eccentric  to  the  south,  rises  the  cone  of  Vesuvius  to  a  height  of 
about  1,300  meters,  the  product  of  the  long  series  of  eruptions  since  79  A.  D.  The 
valley  between  the  Somma  ring  and  the  cone  of  Vesuvius  is  called  the  Atrio  del 
Cavallo. 

As  is  well  known,  the  Vesbian  Volcano  is  a  typical  strato- volcano,  composed  of 
lava  flows  and  interbedded  tuffs,  which  are  deeply  scored  by  radial  erosion  valleys 
on  the  outer  slopes  of  Somma.  The  total  area  covered  by  the  eruptive  products  of 
the  volcano,  exclusive  of  the  thin  beds  of  tuff  which  extend  to  considerable  distances 
around  it,  may  be  estimated  at  300  square  kilometers. 

The  Phlegrean  Fields  are  only  less  well  known  than  Vesuvius,  and  consist  of 
numerous  small  volcanoes,  of  which  Giinther  enumerates  26,  arranged  in  6  groups. 
The  most  important  ones  are:  Pianura  and  Soccavo,  which  are  among  the  earliest; 
Astroni,  Olibano,  Solfatara,  Campiglione,  Cuma,  Averno,  Monte  Nuovo  (which  is 
the  latest  and  was  formed  in  1538),  Monte  di  Procida,  Capo  Miseno,  and  the  small 
islands  of  Nisida,  Procida,  and  Vivara.  None  of  these  volcanoes  is  of  very  large 
size  or  great  altitude,  and  they  have  ejected  very  small  amounts  of  solid  lava, 
the  material  for  the  most  part  being  tuffs.  The  total  area  of  the  Phlegrean  Fields 
has  been  estimated  at  about  130  square  kilometers. 

The  island  of  Ischia,  with  an  area  of  46.5  square  kilometers,  is  somewhat 
pyramidal  in  form,  rising  up  to  the  summit  of  Monte  Epomeo  (792  meters),  which  is 
the  ruin  of  the  original  large  volcano,  of  the  crater  of  which  traces  remain  at  Fon- 
tana.  In  large  part  the  island,  and  especially  the  main  mass  of  the  Epomeo  Vol- 
cano, is  built  up  of  tuffs,  but  lava  flows  are  also  to  be  found.  Flanking  the  Epomeo 
Volcano,  especially  on  the  east  and  north,  are  several  parasitic  cones,  of  which  there 
may  be  mentioned  Monte  Campagnano  on  the  southeast,  Castello  dTschia  on  the 
east  coast,  the  lava  flow  of  L'Arso  (1302  A.  D.),  which,  issuing  from  Le  Cremate 
about  half-way  up  the  east  slope  of  Epomeo,  reaches  the  sea  at  the  northeastern 
corner  of  the  island,  and  Lago  di  Bagno,  Montagnone,  Monte  Rotaro,  and  Monte 
Tabor,  which  lie  along  the  north  coast.  The  extreme  northwest  corner  of  the  island 
is  occupied  by  the  mass  of  Marecocco  and  Zale,  which  may  belong  to  the  eruption 
of  470  B.  c. 

*  From  an  ancient  name  of  Mount  Vesuvius. 


PETROGRAPHY. 
Introduction. 

The  present  section  will  be  devoted  to  the  purely  petrographic  characters, 
that  is,  to  the  descriptions  of  the  rocks  themselves,  including  their  magmatic  or 
chemical,  modal  or  mineralogical,  and  textural  characters.  While  some  of  the  rocks 
dealt  with  are  well  known  to  all  petrographers,  or  have  been  already  described 
according  to  the  prevailing  systems,  others  are  here  discussed,  at  least  in  detail,  for  the 
first  time.  But  in  order  to  secure  uniformity,  all  the  types  which  have  come  under 
my  observation  will  be  described,  irrespective  of  whether  they  have  previously 
appeared  in  literature  or  not.  This  is  essential,  because  the  older  descriptions,  based 
as  they  are  on  primarily  qualitative  principles  of  classification,  are  inadequate  for 
the  description  and  discussion  of  the  region  according  to  the  newer  system.  Fur- 
thermore, one  of  the  objects  of  this  paper  being  the  illustration  of  the  application 
of  the  recently  proposed  quantitative  system,  the  more  rocks  that  are  described, 
especially  if  of  well-known  types,  the  better  will  the  paper  serve  its  purpose.  The 
tuffs  of  the  region  will  not  be  discussed. 

As  the  specimens  examined  and  the  rock  types  to  be  described  are  very  numer- 
ous, the  descriptions  will  be  as  brief  as  may  be  consistent  with  their  purpose.  Minute 
details  and  certain  mineral  and  textural  peculiarities,  such  as  the  extinction  angles 
of  the  feldspars  and  other  minerals,  schemes  ofp  leochroism,  the  details  of  zonal 
structure  and  of  the  arrangement  and  character  of  inclusions,  etc.,  will  either  be 
omitted  or  only  briefly  alluded  to.  While  undoubtedly  of  great  value,  such  minutiae 
belong  rather  to  the  descriptions  of  single  specimens,  and  considerations  of  space 
and  time  would,  in  any  case,  lead  to  their  omission  here.  Direct  reference  to  previous 
work  or  observations  will  not  often  be  made,  but  further  details  may  be  looked 
for  in  the  references  given  in  the  bibliography. 

On  the  other- hand,  the  descriptions  will  be  much  more  quantitative  than  has 
hitherto  been  attempted,  in  accordance  with  the  fundamental  principles  of  the 
classification  and  nomenclature  adopted  here.  The  necessity  of  this  in  the  prem- 
ises is  apparent,  but  the  somewhat  laborious  and  time-consuming  work  involved 
by  this  method  of  treatment,  as  well  as  the  difficulty  of  applying  microscopical 
measurements  to  rocks  of  such  fine  grain  as  are  many  of  these,  have  limited  their 
application,  at  least  in  an  exact  way,  to  those  rock  specimens  which  were  analyzed 
chemically  and  which  serve  as  types.  The  others  have  been  classified  by  reference 
to  and  comparison  with  these  type  specimens,  supplemented  in  some  necessary 
cases  by  measurements  under  the  microscope.  On  this  account  the  reference  of 
some  of  the  unanalyzed  rocks  to  certain  classificatory  positions,  or  their  classifica- 
tion, is  not  altogether  certain.  It  was  found,  however,  that  practice  in  the  compari- 
son of  rocks  from  the  quantitative  point  of  view,  and  the  gradual  assumption  of  a 
mental  attitude  of  regarding  them  quantitatively,  made  the  discrimination  and 

9 


10  THE  ROMAN  COMAGMATIC  REGION. 

decision  gradually  easier,  so  that  in  the  final  general  survey  of  all  the  specimens  and 
their  thin  sections  there  were  very  few  cases  where  any  change  in  the  assigned  posi- 
tion seemed  to  be  justified,  and  few  attributions  which  were  seriously  in  doubt. 

CHEMICAL  ANALYSES. 

As  a  knowledge  of  the  chemical  composition  of  igneous  rocks  is  fundamental 
to  their  classification  according  to  the  system  adopted  here,  especial  attention  was 
paid  to  their  chemical  analysis.  In  the  selection  of  appropriate  specimens  for  this 
the  endeavor  was  made  to  have  them,  as  far  as  possible,  representative  of  the  various 
kinds  of  magma  found  in  the  region — those  which  are  common  as  well  as  those  which 
are  of  rare  occurrence.  Furthermore,  it  was  attempted  to  have  the  analyses  repre- 
sentative of  as  many  as  possible  of  the  different  magmas  found  in  each  district,  so  as 
to  furnish  a  clue  to  the  magmatic  variations  in  each ;  and  again  to  have  analyses 
of  some,  at  least,  of  the  diverse  modes  and  textures  which  each  magma  might  assume 
on  solidification.  These  various  aims  could  not  all  be  completely  attained,  some 
magmas  being  represented  by  many  analyses,  while  of  others  equally  as  abundant 
but  few  were  made.  At  the  same  time,  the  analyses  here  presented  are  so  numer- 
ous, and  cover  such  a  variety  of  rocks  of  widely  diverse  chemical,  modal,  and  tex- 
tural  characters,  that  they  may  be  deemed  an  adequate  basis  for  a  discussion  involv- 
ing the  chemistry  of  the  rocks. 

There  exist  in  the  literature  numerous  analyses  of  the  rocks  of  the  Roman 
Region,  especially  those  of  the  Vulsinian  and  Campanian  districts.  Of  these  older 
analyses  only  a  few  have  been  found,  on  careful  consideration,  to  be  adequate  for 
modern  use  either  as  regards  accuracy  or  completeness.  A  large  series  of  analyses 
of  Vulsinian  rocks  are  not  only  incomplete  in  the  determination  of  minor  constitu- 
ents, but,  as  will  be  shown  later,  are  so  hopelessly  inaccurate,  especially  in  the  figures 
for  the  alkalies,  that  they  may  all  be  rejected  without  hesitation.  The  older  anal- 
yses of  the  Vesuvian  lavas,  while  in  part  much  more  accurate  in  the  determination 
of  the  main  constituents,  are  faulty  on  the  score  of  completeness,  in  the  absence  of 
determinations  of  the  minor  ones,  as  titanium  and  phosphorus.  In  these  rocks,  as 
is  seen  elsewhere,  these  usually  negligible  constituents  assume  a  very  considerable 
importance  in  classification,  so  that  we  can  not  avail  ourselves  here  of  this  otherwise 
adequate  and  useful  older  work,  except  in  a  general  way. 

Most  of  the  analyses  made  and  published  several  years  ago  by  myself  have 
been  incorporated,  as  new  analyses  and  redeterminations  have  shown  them  to  be 
reliable.  In  some  cases  they  have  been  corrected  by  redeterminations  of  some 
of  the  constitutents,  as  well  as  rendered  more  complete  by  the  determination  of  minor 
constituents  in  which  they  were  lacking,  especially  titanium  and  phosphorus.  Un- 
less otherwise  stated,  this  has  been  carried  out  with  material  remaining  from  the 
original  analysis.  But  the  majority  of  the  analyses  presented  in  this  paper  have 
been  made  by  me  especially  for  the  present  investigation.  In  making  them,  as  well 
as  in  the  determinations  which  were  needed  to  bring  the  older  analyses  up  to  modern 
standards  of  completeness,  the  methods  employed  were  those  advocated  by  Hille- 


PETROGRAPHY. 


II 


brand*  and  myself  ,f  and  which  long  experience  has  shown  to  be  reliable.  In  many 
cases  the  rarer  and  less  often  looked-for  constituents,  as  baryta  and  zirconia,  were 
estimated,  with  results  which  are  of  considerable  interest.  In  the  statements  of  the 
analyses  the  molecular  ratios  are  given  in  a  column  immediately  following  that  of 
the  percentage  amounts. 

CLASSIFICATION. 

The  classification  adopted  here  is  the  quantitative  one  recently  proposed,!  a 
description  of  which  it  seems  unnecessary  to  give.  Comparatively  little  literature 
has  as  yet  appeared  expressed  in  the  terms  of  the  new  system,  and  little  in  the  way 
of  describing  or  establishing  types  according  to  the  suggestions  made,  so  that  the 
present  paper  may  be  regarded  as  illustrative  of  the  practical  application  of  the  new 
classificatory  principles  and  nomenclature  to  the  actual  study  of  rocks  and  the 
discussion  of  petrological  problems. 

It  is  a  somewhat  difficult  matter  for  the  petrographer  to  change  his  conceptions 
of  rocks  so  as  to  conform  to  the  new  requirements  and  to  regard  rocks  primarily  as 
magmas  of  which  they  are  but  the  solidified  forms,  and  more  especially  to  keep 
the  importance  of  the  quantitative  relations  constantly  in  mind.  This  has  proved 
true  in  my  own  case,  and  it  has  required  considerable  effort  at  times  to  keep  to  the 
narrow  and  untrodden  path  of  a  quantitative  system  based  primarily  on  chemical 
composition,  and  to  refrain  from  treading  the  broad  road,  easy  from  much  use  in 
the  past,  of  qualitative  systems  based  on  mineral  composition  and  texture.  To 
what  extent  this  effort  to  adapt  the  mental  attitude  to  the  new  requirements  is  suc- 
cessful, it  will  be  easier  for  the  reader  than  for  the  writer  to  judge. 

Magmatic  Divisions  Represented  in  the  Roman  Region. 


CLASS. 

ORDER. 

RANG. 

SUBRANG 

I.  Persalane  .... 

5.   Canadare  
6.    Russare  

(  i.   Nordmarkase 
'  2.   Pulaskase  
(  i.   Miaskase  

3.   Phlegrose, 
(  2.   Vulsinose, 
(  3.   Pulaskose, 
3.    Beemerose, 

I-   5-  I-  3-  § 

I.     5.  2.  2. 
I.     5.  2.  3. 
I.     6.   I.  3. 

7.   Tasmanare  .  .  . 
4.    Austrare  .... 

I  2.   Viezzenase  .... 
i.    Laugenase.  .  .  . 
3     Tonalase  

3.   Procenose, 
3.   Appianose, 
3.    Harzose, 

I.   6.  2.  3. 

I-     7-1.3- 
II.    A    3.  3. 

II    Dosalane  . 

5.    Germanare.  .  .  . 

(  2.   Monzonase.  .  .  . 
(  3    Andase   

(  2.    Ciminose, 
I  3.   Monzonose, 
(  2.   Auruncose, 

II.     5.  2.  2. 
II.     5.2.3. 

II.  5-  3-  *• 

6    Norgare  

2.    Essexase  

\  3.   Shoshonose, 
2.   Vicose, 

n-   5-  3-  3- 

II.     6    2.  2. 

7.   Italare  

2.   Vulturase  

2.   Braccianose, 

II.     7.  2.  2. 

8.    Campanare  .  .  . 

2.   Vesuvase  

2.   Vesuvose, 

II.     8.  2.  2. 

III.   Salfemane  .  .  . 

(  7.   Kamerunare  .  . 

(  2.    Kamerunase  .  . 
(  3.   Etindase  

2.   Jugose, 
2.    Fiasconose, 

III.     7.  2.  2. 
III.    732. 

(  8.    Bohemare  .... 

2.    Albanase  

2.   Albanose, 

III.     8.  2.  2. 

*  W.  F.  Hillebrand,  Some  Principles  and  Methods  o}  Rock  Analysis,  Bull.  U.  S.  Geol.  Surv.  No.  176,  1900 

t  H.  S.  Washington.  Manual  oj  the  Chemical  Analysis  of  Rocks,  New  York  (1904). 

$  Cross,  Iddings,  Pirsson,  and  Washington,  Quantitative  Classification  oj  Igneous  Rocks  (Chicago,   1003). 
Cf.  Journal  0}  Geology,  X  (1002),  pp.  555-600. 

§  For  the  explanation  of  these  symbols  to  indicate  the  magmatic  position,  see  H.  S.  Washington,  Prof.  Paper 
U.  S.  Geol.  Surv.  No.  28,  1904,  p.  13. 


12  THE  ROMAN  COMAGMATIC  REGION. 

• 

In  the  terms  of  the  quantitative  classification  the  preceding  magmatic  divi- 
sions, as  far  as  subrang,  are  represented  in  the  Roman  Region.  They  include  3 
classes,  10  orders,  14  rangs,  and  17  subrangs.  There  are  probably  a  few  more 
which  are  not  discussed  in  this  paper,  but  the  amounts  of  these  are  probably  small, 
and  those  given  here  undoubtedly  embrace  the  great  majority  of  the  most  impor- 
tant and  most  abundant  of  the  Roman  magmas. 

DESCRIPTIONS  AND  NAMES  OF  ROCKS. 

The  rocks  will  be  described  under  the  heads  of  the  various  types  to  which  they 
belong,  a  type  being  understood  to  mean  a  standard  rock  with  a  certain  assemblage 
of  characters,  accurately  and  quantitatively  described  as  regards  the  chemical  com- 
position, norm,  mode,  texture,  and  the  textural  disposition  of  the  constituent  min- 
erals. Rocks  will  be  considered  to  belong  to  the  same  type  when  they  are  quite 
or  almost  identical  in  these  respects,  so  much  so  that  they  may  be  mistaken  for  one 
another,  or  might  be  parts  of  the  same  mass,  as  suggested  in  the  publication  of 
the  Quantitative  System  (p.  179).  As  a  practical  matter  the  identity  can  not  be 
absolute  in  all  cases,  and  a  certain  amount  of  latitude,  or  very  slight  divergence 
from  the  type,  must  be  allowed.  Otherwise  the  number  of  types  will  be  almost  as 
great  as  the  number  of  the  occurrences  or  of  the  hand  specimens  collected.  This 
latitude  is  only  reasonable,  and  that  it  is  allowable  is  shown  by  the  practice  in 
botany,  where,  for  instance,  individual  plants  are  referred  to  the  same  species, 
although  there  are  slight  differences  in  minor  and  unimportant  points,  such  as  the 
lengths  of  the  leaves,  the  heights  in  different  habitats,  etc. 

The  name  of  a  rock,  then,  according  to  the  quantitative  system,  and  as  applied 
throughout  this  paper,  will  be  considered  to  be  the  type  name — that  is,  the  name 
of  the  subrang  qualified  by  a  typal  adjective  or  one  indicating  the  type  to  which 
the  rock  belongs.  This  typal  adjective  is  formed  according  to  the  published  sug- 
gestion by  the  use  of  a  root  derived  from  a  geographical  locality  and  the  termina- 
tion al.  Such  a  name  expresses  as  concisely  as  possible,  and  without  ambiguity, 
or  at  least  with  only  very  narrow  limits  of  variation,  all  the  inherent  characters  of 
a  rock,  the  subrang  name  indicating  its  chemical  character  and  systematic  position 
in  the  classification,  and  the  type  adjective  its  color,  modal  and  textural  charac- 
ters, the  textural  disposition  of  the  minerals,  etc. 

For  the  purpose  of  correlation  with  the  prevailing  systems,  and  for  the  benefit 
of  those  who  are  unacquainted  or  unfamiliar  with  the  quantitative  system,  the  names 
by  which  the  rocks  would  be  called  in  the  prevalent  nomenclature  are  also  used 
throughout  the  paper,  in  brackets  [  ],  while  the  symbols  of  the  subrang  will  be  placed 
in  parentheses  (  )  when  it  is  deemed  advisable  to  add  them  to  the  quantitative 
rock  name.  But  it  must  be  remembered  that  these  symbols  do  not  form  any  part 
of  the  name,  and  are  used  only  to  indicate  the  magmatic  position  on  account  of  the 
unfamiliarity  of  the  new  terms. 

In  assigning  the  names  of  the  prevailing  nomenclature,  the  usage  of  Zirkel  in 


PETROGRAPHY.  13 

regard  to  leucite-trachyte  and  leucite-phonolite  has  been  followed  rather  than  that 
of  Rosenbusch,  for  reasons  discussed  elsewhere.  The  term  "leucite-trachyte," 
therefore,  as  used  in  this  paper,  implies  a  rock  composed  essentially  of  leucite  and 
orthoclase,  while  a  "leucite-phonolite"  is  one  composed  of  the  same  two  minerals 
with  nephelite  in  addition,  in  both  cases  the  presence  of  subordinate  amounts  of 
alferric  and  other  minerals  being  allowed.  It  has  sometimes  been  difficult  to  decide 
between  two  names,  as,  for  instance,  between  "leucite-trachyte"  and  "leucite- 
tephrite,"  or  "  leucite- tephrite "  and  "leucitite,"  as  both  labradorite  and  ortho- 
clase may  be  present  in  the  former  case,  or  a  little  labradorite  alone  in  the  latter,  in 
both  cases  with  much  leucite.  In  all  such  cases  the  name  finally  selected  was  based, 
as  far  as  possible,  on  the  quantitative  estimation  of  the  mode,  modified  in  some 
instances  by  the  greater  prominence  of  one  or  the  other  mineral. 

A  list  is  appended  of  the  various  rock  types  which  have  been  observed  and 
described  in  the  region,  with  their  corresponding  names  according  to  the  prevailing 
systems,  so  that  comparison  between  the  two  may  be  rendered  more  easy.  In  the 
prevailing  names  I  have  followed  the  trend  of  some  modern  authors  in  further  quali- 
fying the  broader  and  more  common  names  by  the  use  of  type  names.  This  would 
seem  to  be  reasonable,  as  it  is  certainly  advisable  to  be  able  to  discriminate,  even  in 
the  customary  nomenclature,  between,  for  instance,  three  such  different  "leucite- 
tephrites"  as  those  of  the  Viterbo  type,  with  its  large  and  numerous  leucite  pheno- 
crysts,  that  of  the  Orvieto  type,  without  any  phenocrysts,  and  that  of  the  Tavolato 
type,  with  large  leucite  phenocrysts,  abundant  hatiyne,  little  feldspar,  and  very 
different  chemical  composition. 

In  two  or  three  instances  new  names  have  been  proposed  for  use  in  the  pre- 
vailing system  of  nomenclature.  This  has  been  done  with  caution,  and  only  in  those 
cases  where  they  seemed  to  be  especially  necessary  or  useful,  on  account  of  the  pecu- 
liar chemical,  mineralogical,  or  textural  character  of  the  type,  as  with  the  abundant 
and  highly  characteristic  leucite-trachytes  and  leucite-tephrites  with  the  viterboid 
habit  marked  by  the  numerous  large  leucite  phenocrysts,  or  the  peculiar  leucite- 
tephrite  of  Tavolato,  with  its  abundant  large  hauynes  and  peculiar  chemical  com- 
position, which  has  little  in  common  with  the  leucite-tephrites  as  usually  understood. 
In  one  instance  an  old  and  forgotten  rock  name  has  been  revived — that  of  cecilite 
(Cordier),  for  the  highly  melilitic  leucitites. 

It  is  seen  that  many  of  the  prevailing  rock  names  cover  wide  variations  in  tex- 
ture and  magmatic  characters — a  point  which  will  be  more  fully  appreciated  by 
referring  to  the  descriptions  which  follow.  And  it  is  also  evident  that  some  of  the 
subrang  names  are  applied  to  rocks  which  differ  much  in  their  modes  and  textures, 
a  consequence  of  the  fact  that  the  quantitative  system  is  based  primarily  on  the 
chemical  composition  of  the  rock,  and  that  the  modal  and  textural  characters  are 
regarded  as  of  less  importance,  they  being  expressed  by  the  type  name.  A  study  of 
the  table  from  this  point  of  view  is  very  instructive,  and  the  relation  between  the  two 
systems  should  be  borne  in  mind  when  reading  the  descriptions  of  the  various  types. 


THE  ROMAN  COMAGMATIC  REGION. 


Rock  Types  Occurring  in  the  Roman  Region. 


SYMBOL. 


NAME  IN  QUANTITATIVE  SYSTEM. 


NAME  IN  PREVAILING  SYSTEM. 


1.5.  1.4-3. 
I-5-I-3-.- 

I.  5.  2.  2... 
I.  5.   2.   2-3  . 

I.  6-5.  2.  3  . 

I.  6.   I.  3  .  .  . 

II-I.  i.  7.  i-3- 

II.  4.3-3- •• 

I-II.  5.   2.   2.    . 
II.  5.  2.   2... 

II.  6-5.   2.   2. 
II.  5.   2.  3.   .  . 

II.  5.  2-3.   2  . 
11.5-3.2... 

n.  5. 3. 3... 

II.  6.  2.   2.   .  . 
II.   7.   2.   2.   .  . 


II.  8-7.   2.   2. 

II.  8.  2.  2... 
III.  8-7.  2.  2. 
m.7.3.2... 


Ischial  nordmarkose-phlegrose..  . 

Cumal  phlegrose 

Rotaral  phlegrose 

Arsal  vulsinose 

Bolsenal  vulsinose 

Viterbal  vulsinose 

Pallanzanal  vulsinose 

Bolsenal  vulsinose-pulaskose 

Paglial  procenose-pulaskose 

Sabatinal  beemerose 

Tavolatal  janeirose-appianose.  . 

Sorianal  harzose 

Arsal  vulsinose-ciminose 

Fiescolal  ciminose 

Viterbal  ciminose 

Bagnoreal  ciminose 

Martinal  vicose-ciminose 

Arsal  monzonose 

Bolsenal  monzonose 

Foglianal  ciminose-auruncose  . . . 

Teanal  ciminose-auruncose 

Orvietal  auruncose 

Monfinal  shoshonose 

Foglianal  vicose 

Bagnoreal  vicose 

Orvietal  vicose 

Vesbal  braccianose 

Sommal  braccianose 

Galeral  braccianose 

Hernical  braccianose 

Atrial  braccianose 

Scalal  vesuvose-braccianose 

Romal  vesuvose 

Galeral  albanose-jugose 

Fiordinal  fiasconose 

Romal  albanose 

Saccal  albanose 

Boval  albanose . . 


Augite-trachyte  (Ischia  type). 
Phonolitic  trachyte  (Cuma  type). 
Trachyte  obsidian  (Rotaro  type). 
Vulsinite  (Arso  type). 
Vulsinite  (Bolsena  type). 
Leucite-trachyte  (Viterbo  type). 
Leucite-trachyte  (Pallanzana  type). 
Vulsinite  (Bolsena  type). 
Leucite-trachyte  (Paglia  type). 
Leucite-phonolite  (Sabatino  type). 
Leucite-tephrite  (Tavolato  type). 
Biotite-latite  (Soriano  type). 
Vulsinite  (Arso  type). 
Ciminite  (Fiescoli  type). 
Leucite-trachyte  (Viterbo  type). 
Leucite-trachyte  (Bagnoreal  type). 
Leucite-tephrite  (San  Martino  type). 
Vulsinite  (Arso  type). 
Vulsinite  (Bolsena  type). 
Leucite-tephrite  (Viterbo  type). 
Leucite-trachyte  (Teano  type). 
Leucite-tephrite  (Orvieto  type). 
Biotite-latite  (Monfina  type). 
Leucite-tephrite  (Viterbo  type). 
Leucite-tephrite  (Bagnorea  type). 
Leucite-tephrite  (Orvieto  type). 
Leucite-tephrite  (Vesuvius  type). 
Leucite-tephrite  (Somma  type). 
Leucitite  (Galera  type). 
Leucitite  (Hernici  type). 
Leucite-tephrite  (Atrio  type). 
Leucite-tephrite  (Scala  type). 
Leucitite  (Rome  type). 
Leucitite  (Galera  type). 
Leucite-basanite  (Fiordine  type). 
Leucitite  (Rome  type). 
Leucitite  (Sacco  type). 
Leucitite  (Bove  type),  cecilite. 


In  the  descriptions  of  the  types  the  order  in  which  they  will  be  taken  up  is  that 
of  their  magmatic  position,  by  class,  order,  rang,  and  subrang.  The  divisions 
below  subrang — that  is,  grads  and  subgrads — will  not  be  regarded  or  discussed,  as 
it  is  thought  that  there  are  sufficient  new  terms  used  in  this  paper  without  adding 
to  them  the  many  complications  and  more  numerous  names  which  the  introduction 
of  these  minor  divisions  would  involve.  It  was  also  thought  desirable  to  determine 
by  actual  practice  whether  the  refinement  in  classification  implied  by  the  use  of 
grads  and  subgrads  in  the  dosalanes  and  salfemanes  is  strictly  necessary,  or  whether 
the  general  needs  of  petrographic  description  and  discussion  can  not  be  met  without 
their  use.  In  a  perfect  system,  carried  out  to  the  last  details,  they  are  undoubtedly 
logical  and  necessary,  but  for  practical  purposes  at  present  it  would  seem  advisable 
not  to  discard  them,  but  to  avoid  the  complications  that  they  introduce,  and  to 


PETROGRAPHY.  15 

reserve  them  for  the  more  detailed  and  special  investigations.  From  examination 
of  the  subsequent  pages,  and  when  it  is  considered  that  the  rocks  of  any  one  region 
have  certain  chemical  characters  in  common,  it  is  fairly  clear  that  grads  and  subgrads 
can  be  dispensed  with  in  great  part  for  general  purposes,  and  that  the  use  of  type 
adjectives  will  replace  them  advantageously  to  a  large  extent. 

In  each  subrang  the  general  order  will  be  that  those  types  whose  modes  are 
most  normative  will  be  first  described,  followed  by  those  which  are  abnormative, 
in  each  case  the  most  coarsely  granular,  or  those  with  the  largest  and  most  abundant 
phenocrysts,  preceding  the  less  porphyritic  or  aphyric  ones.  The  hyaline  types 
will  be  the  last  to  be  described. 

In  a  number  of  cases  the  rock  habits  will  be  named — that  is,  those  more  gen- 
eral features  of  mode  and  texture  or  both  in  which  rocks  may  resemble  each  other 
without  belonging  to  the  same  type.  The  habits  named,  however,  are  only  a  few, 
and  are  those  which  seem  to  be  most  common  to,  or  characteristic  of,  the  rocks  of 
the  region. 

In  the  statement  of  the  norm  one  modification  from  the  original  has  been 
introduced,  the  result  of  a  joint  discussion  of  the  subject  on  the  part  of  all  the  authors 
of  the  quantitative  system.  This  has  to  do  with  the  normative  minerals  sodalite 
and  noselite.  Without  going  into  details  here,  it  was  thought  advisable  to  modify 
these  to  the  extent  of  splitting  them  up  and  stating  in  the  norm  the  amounts  of 
halite  (NaCl)  with  the  symbol  HI,  and  of  thenardite  (Na2SO4)  with  the  symbol 
Th,  instead  of  sodalite  and  noselite.  The  soda  which  was  previously  combined 
with  the  sodium  chloride  and  sulphate  remains  with  the  rest  in  calculating  the  norm, 
and,  if  necessary,  is  distributed  between  albite  and  nephelite  in  the  usual  way. 
In  some  cases  a  change  in  the  position  of  the  rock  as  regards  the  order  is  thus 
brought  about,  but  this  will  happen  only  when  the  amounts  of  chlorine  or  sulphuric 
anhydride  are  large;  and,  as  a  matter  of  fact,  it  was  found  that  in  the  present  series 
the  systematic  position  of  the  rock  was  not  altered,  nor  the  relative  amounts  of 
feldspar  and  lenad"  changed  in  the  slightest,  in  any  of  the  few  cases  in  which  the 
sodalite  minerals  are  present.  An  advantage  of  this  method  of  procedure  is  that  it 
minimizes  the  influence  of  the  small  amounts  of  Cl  and  SO3  usually  found,  which 
is  very  great  if  they  bind  up  in  the  norm  a  much  greater  amount  of  soda  and  silica. 
This  matter  will  be  discussed  in  a  forthcoming  joint  paper,  along  with  other  modi- 
fications of  and  additions  to  the  system. 

THE  FORMAL  DESCRIPTIONS. 

Following  the  descriptions  of  the  different  types,  which  are  couched  in  the  usual 
petrographical  terms,  except  such  as  belong  to  the  phraseology  of  the  quantitative 
system,  are  what  may  be  called  formal  descriptions  of  the  types.  These  aim  to  give 
a  concise  but  complete  description  oi  the  type,  both  qualitative  and  quantitative, 
which  may  be  regarded  as  standard,  and  from  which  one  may  judge  whether 
another  rock  belongs  to  the  same  type  or  not,  certain  narrow  departures  being 


1 6  THE  ROMAN  COMAGMATIC  REGION. 

regarded  as  allowable.    They  are  based  on  the  descriptions  of  botany  and  zoology, 
and  follow  the  plan  of  these  in  general,  mutatis  mutandis. 

These  have,  both  in  form  and  in  language,  been  adopted  after  consultation  with 
my  colleagues  in  the  quantitative  system,  though  they  may  be  regarded  as  possibly 
not  definitive  or  final,  but  as  an  attempt  at  a  more  precise  and  concise  statement  of 
the  characters  of  igneous  rocks  such  as  is  demanded  by  the  new  system  of  classifi- 
cation for  the  establishing  of  types.  Many  of  the  terms  used  in  these  formal 
descriptions  are  some  which  have  recently  been  adopted  by  ourselves,  and  which  will 
be  published  by  us  elsewhere  in  a  paper  setting  forth  some  additions  to  the  quantita- 
tive system  of  classification.  Most  of  these  will  be  self-explanatory. 

The  general  plan  on  which  these  formal  descriptions  are  arranged  is  as  follows: 
The  megascopic  characters,  or  those  determinable  in  the  field,  will  be  given  first; 
the  more  general,  or  those  which  are  discernible  at  a  greater  distance  from  the  eye, 
being  stated  before  the  more  particular.  If  phenocrysts  are  present,  they  will  be 
named  and  briefly  described  megascopically  in  the  order  of  their  abundance.  The 
statement  of  the  specific  gravity  of  the  analyzed  specimen  belongs  in  this  part  of  the 
description. 

Next  follows  the  description  of  the  characters  discernible  only  in  the  thin  sec- 
tion— the  microscopic  characters.  Here  also  the  more  general  features  of  texture, 
as  being  the  more  readily  discernible,  come  first,  in  the  order  of  characters  pertain- 
ing to  crystallinity,  granularity,  and  fabric.  Then  follows  a  succinct  statement  of 
the  mode,  the  minerals  present  being  given  in  the  order  of  abundance,  and,  if  the 
rock  is  porphyritic,  the  phenocrystic  minerals  preceding  those  which  belong  to  the 
groundmass.  After  this  come  the  descriptions  of  the  several  minerals,  given  in  the- 
order  of  their  statement  in  the  norm,  though  the  alferric  minerals,  which  do  not 
belong  to  the  norm,  are  given  in  the  order,  augite,  hornblende,  biotite,  garnet,  and 
precede  the  femic  minerals.  The  characters  of  the  several  minerals  are  stated  in 
the  order  of  their  importance,  their  relative  amount  in  the  rock  (by  weight,  not  by 
volume),  their  size,  crystal  development,  shape,  twinning  phenomena,  color,  inclu- 
sions, and  arrangement.  A  detailed  statement  of  the  optical  and  other  characters,, 
as  extinction  angles,  pleochroism,  zonal  variation,  etc.,  will  not  be  attempted,  nor 
are  data  given  as  to  the  order  of  formation,  though  these  last  may  well  be  considered 
to  have  a  place  in  the  formal  description. 

It  is,  of  course,  understood  that  the  figures  expressing  relative  amounts  and 
sizes,  which  have  been  arrived  at  by  study  under  the  microscope,  are  averages  merely, 
and  that  other  rocks  may  depart  from  one  or  more  of  them  within  reasonable 
limits  and  still  be  considered  as  belonging  to  the  same  type. 

These  formal  descriptions  are  pen  pictures  of  the  rocks — skeletons  which  the 
imagination  of  the  reader  must  clothe  with  reality,  but  which  furnish  in  concrete 
form  the  absolute  and  relative  characters  with  which  other  investigators  may  com- 
pare other  rocks  in  the  future.  They  are  intended  primarily  for  reference,  sup- 
plementing and  stating  the  facts  of  the  ordinary  descriptions. 


PETROGRAPHY.  17 

TRANSITIONAL  TYPES. 

In  the  list  of  types  given  above,  as  well  as  in  the  descriptions  which  follow,  there 
will  be  noticed  a  considerable  number  of  types  which  fall  almost  on  the  borders 
between  two  or  more  magmatic  divisions,  or  are  "transitional."  Their  occurrence 
is  of  importance  as  illustrating  a  feature  of  the  quantitative  classification  which,  as 
has  been  said  elsewhere,*  is  not  peculiar  to  it,  but  is  inherent  in  the  character  of 
igneous  rocks,  so  that  no  "natural"  division  lines  exist,  and  those  which  are  selected 
must  be  arbitrary,  in  the  nature  of  the  case.  As  has  been  said,  such  transitional 
types  are  just  as  important  as  those  which  fall  at  the  centers  of  the  various  divi- 
sions— a  fact  clearly  brought  out  in  the  present  region,  where  transitional  types  are 
very  numerous,  and  to  which  belong  some  of  the  most  abundant  and  characteristic 

§  rocks. 
a 

To  emphasize  this  fact  of  the  equality  in  importance  of  "central"  and  "tran- 
sitional" types,  the  last  have  in  nearly  every  case  been  described  separately,  even 
though  this  involved  an  increase  in  the  number  of  types. 

In  this  connection  the  question  naturally  arises:  How  close  to  the  border  of  a 
given  division  must  a  magma  fall  to  be  considered  transitional?  The  answer  to 
this  is  debatable,  and  one  which  has  been  under  consideration  by  the  proposers  of 
the  quantitative  system  for  some  time.  While  no  definite  conclusion  has  been 
reached  as  yet,  it  would  seem  that  the  decision  must  rest  largely  upon  the  petrog- 
rapher  himself  in  any  given  case.  The  need  for  arbitrary  boundaries  is  not  felt 
here  as  in  the  establishment  of  the  definite  magmatic  divisions,  and,  furthermore, 
it  is  evident  that,  were  the  attempt  made  to  establish  rigid  borders  within  or  outside 
of  which  rocks  should  be  said  to  be  or  not  to  be  transitional,  the  difficulty  would 
only  be  shifted,  not  eliminated  entirely.  Thus  it  might  easily  happen  that  the 
carrying  out  of  the  molecular  ratios  to  four  instead  of  three  decimal  places,  or  the 
determination  and  introduction  into  the  calculation  of  such  minor  constituents  as 
zirconia,  baryta,  or  strontia,  could  change  the  type  from  a  transitional  to  a  non- 
transitional  one,  or  vice  -versa. 

The  treatment  adopted  here  is  rather  a  tentative  than  a  final  one,  and  in  places 
may  not  be  wholly  consistent.  As  a  general  rule,  a  type  or  a  magma  has  been  con- 
sidered transitional  when  the  ratio  in  question  differs  from  that  of  the  border  line  by 
±0.05  or  ±0.10.  Thus  where  the  border  ratio  is  7:1,  the  magma  is  regarded  as 
transitional  if  the  ratio  falls  between  6.90  and  7.10.  If  the  ratio  of  the  border  is 
5:3  =  1.666  +  ,  the  magma  is  regarded  as  transitional  if  it  falls  between  1.60  and 
1.70,  a  difference  here  of  only  about  ±0.05,  as  the  border  ratio  is  less  than  in  the 
other  case.  But  these  are  only  in  the  nature  of  general  and  tentative  suggestions, 
and  this  seems  to  be  one  of  the  features  of  the  quantitative  classification  which  it 
were  best  to  leave  somewhat  indefinite,  just  as  are  the  limits  of  identity  or  simi- 
larity in  mode  and  texture  which  may  define  a  type,  and  which  are  briefly  dis- 
cussed on  another  page. 

*  Cross,  Iddings,  Pirsson,  and  Washington,  op.  tit.,  pp.  121,  166,  231;  Iddings,  Prof.  Paper  U.  S.  Geol.  Surv. 
No.  18,  1903,  p.  69;  Pirsson,  Bull.  U.  S.  Geol.  Surv.  No.  237,  1003,  p.  120. 


l8  THE  ROMAN  COMAGMATIC  REGION. 

DETERMINATION  OF  MINOR  CONSTITUENTS. 

The  remarks  above  lead  to  the  consideration  of  another  matter  of  some 
importance  which  is  clearly  brought  out  in  the  following  pages,  namely,  the  impor- 
tance in  the  quantitative  system  of  determining  the  chemical  constituents  which 
are  usually  regarded  as  minor,  or  at  least  some  of  them.  It  will  be  observed  in  a 
number  of  cases  that  the  classificatory  position  of  a  rock  is  changed  by  the  deter- 
mination of  and  introduction  into  the  calculation  of  some  constituent,  as  TiO2  or 
P2OS,  which  is  present  only  in  very  small  amount,  sometimes  less  than  one-half  of 
i  per  cent.  Thus  the  arsal  vulsinose-ciminose  [vulsinite]  of  Vetralla  was  formerly 
placed  in  vulsinose,  but  the  recent  determination  of  0.61  TiO2  and  0.17  P2OS  shows 
that  it  is  properly  in  ciminose;  similarly,  the  determination  of  0.77  P2OS  has  shifted 
the  "leucite-tephrite"  of  La  Scala  from  vesuvose  into  braccianose.  Such  cases  are 
to  be  expected,  and  while  they  may  be  considered  by  some  to  be  a  practical  defect 
in  the  quantitative  system  in  demanding  what  may  seem  to  be  ultra-refinements  in 
the  chemical  analysis,  they  can  not  justly  be  regarded  in  this  light.  This  has  been 
briefly  discussed  elsewhere,*  and  it  was  pointed  out  that  it  is  "not  an  objection,  but 
really  a  very  strong  point  in  favor  of  the  new  system.  For  it  postulates  as  funda- 
mental to  the  classification  the  absolute  necessity  for  only  the  best  class  of  analyti- 
cal work."  And  it  may  be  said  in  addition,  though  it  can  not  be  held  as  an  excuse 
for,  or  as  justifying,  incomplete  analyses,  that  changes  thus  brought  about  by  the 
determination  of  these  minor  constituents  will  usually  happen  only  when  the  rock 
is  transitional  as  to  magmatic  position. 

In  the  present  case,  however,  some  of  the  rocks  involved  are  of  such  a  char- 
acter that  the  determination  of  small  amounts  of  TiO2,  P2OS,  etc.,  or  slight  inac- 
curacies in  the  determination  of  the  main  constituents,  as  silica  or  the  alkalis,  will 
bring  about  very  serious  changes  in  the  magmatic  position.  These  rocks  are  those 
which  belong  to  the  more  lenic  orders  of  dosalane,  braccianose  and  vesuvose,  and 
to  the  salfemanes.  In  the  present  region  these  rocks  are  leucitic  without  exception; 
that  is,  are  dopotassic  and  with  a  deficiency  of  silica.  On  account  of  the  silica 
relations  of  orthoclase  and  leucite  with  64.75  and  S5-°5  Per  cent  of  silica  respectively, 
a  very  slight  difference  in  the  amount  of  silica  or  the  alkalis,  or  in  other  compo- 
nents, may  bring  about  very  decided  changes  in  the  norm,  owing  to  the  readjust- 
ments of  potash  between  the  polysilicate  orthoclase,  KAlSi3Os,  and  the  metasili- 
cate  molecule,  KAlSi2O6- 

In  other  words,  the  limits  of  the  dopotassic  subrangs  of  these  lenic  orders  are 
very  narrow.  This  is  very  clearly  shown  by  Iddingsf  in  his  diagram  of  the  limits 
of  orders  and  rangs  for  theoretically  pure  potash-salic  rocks.  In  this  the  uniformly 
smaller  area  of  the  potassic  subrangs,  as  compared  with  the  purely  sodic  ones,  and 
especially  the  extremely  small  dimensions  attained  by  the  potassic  divisions  as  the 
silica  decreases  and  the  amount  of  the  lenad  leucite  rises,  are  very  clearly  brought 

*  Washington,  Prof.  Paper  U.  S.  Geol.  Surv.  No.  14,  1003,  p.  44. 

t  Iddings,  Prof.  Paper  U.  S.  Geol.  Surv.  No.  18,  1003,  p.  72  and  Plate  II. 


PETROGRAPHY.  19 

out,  and  will  repay  some  study.     This  matter  will  be  discussed  later  in  connection 
with  the  formation  of  leucite. 

I.  5.  1.  4-3.  Ischial  Nordmarkose-Phlegrose  [Augite-Trachyte,  IschiaType]. 

Megascopic  characters. — Rocks  of  this  type  are  light  gray,  rough  in  feel,  and 
highly  porphyritic.  The  most  abundant  phenocrysts  are  of  feldspar,  which  con- 
stitute about  30  per  cent  of  the  rock.  These  are  tabular  or  stout  prismatic,  from 
10  to  20  mm.  long,  colorless,  and  with  a  highly  vitreous  luster.  From  the  absence 
of  twinning  striations  they  would  be  considered  to  be  orthoclase.  Small  prismatic 
phenocrysts  of  black  augite  and  grains  of  magnetite  are  very  rare,  not  making  up 
more  than  i  or  2  per  cent  of  the  rock.  The  ground  mass  is  very  light  gray  and 
phanerocrystalline,  but  very  fine-grained.  It  is  obviously  composed  almost  exclu- 
sively of  salic  minerals,  presumably  feldspars. 

Microscopic  characters. — The  thin  sections  show  the  following  minerals:  Alkali 
feldspar  very  abundant,  with  less  augite  and  magnetite,  and  accessory  lavenite  and 
sodalite.  The  texture  is  typically  holocrystalline,  and  with  a  characteristic  trachy- 
tic  fabric. 

The  feldspar  phenocrysts  are  of  soda-orthoclase  in  thick  subhedral  tables,  tab- 
ular parallel  to  b  (oio),  or  occasionally  stoutly  prismatic  parallel  to  the  a  axis.  They 
are  not  uncommonly  fragmentary,  having  been  broken  during  the  flow.  Although 
their  sodic  character  is  certain  from  chemical  analysis,  the  microscopic  appearances 
so  characteristic  of  soda-orthoclase  are  usually  absent.  They  show,  for  the  most 
part,  a  clear,  even  gray  between  crossed  nicols,  without  the  lamellae  of  microcline, 
microperthitic  intergrowths  of  orthoclase  and  albite,  or  the  common  moire*  appear- 
ance. The  rare  augite  phenocrysts  are  subhedral,  in  stout  prismoids,  and  of  a  very 
pale-gray  color,  or  with  only  an  extremely  faint  tinge  of  green.  Inclusions  are  very 
rare  both  in  these  and  in  the  feldspar  phenocrysts. 

The  groundmass  has  the  typical  trachytic  fabric,  and  usually  with  marked  evi- 
dences of  flow.  It  is  composed  in  very  great  part  of  small  prismoids  of  alkali  feld- 
spar, elongated  parallel  to  the  axis  a.  Small  prismoids  of  colorless  augite  and 
grains  of  magnetite  are  present,  but  only  in  negligible  amounts.  In  some  specimens 
lavenite  occurs  as  a  rare  accessory,  in  small,  yellow,  pleochroic  prismoids,  often 
arranged  in  divergent  clusters;  but  this  mineral  can  not  be  regarded  as  character- 
istic of  the  type.  The  same  is  true  of  sodalite,  which  is  sometimes  present  in  very 
small  amount  in  the  groundmass  as  colorless,  rounded  anhedra,  as  weU  as  in  crevices 
of  the  rock.  No  glass  could  be  detected  in  most  of  the  specimens,  though  it  may  be 
present  to  a  very  slight  extent. 

Chemical  composition. — An  analysis  of  this  type  was  published  some  years  ago, 
and  is  here  repeated  in  more  complete  form.  An  older  analysis  by  Fuchs  is  also 
given  for  comparison. 

The  analysis  is  noteworthy  because  it  shows  that  a  typical  "trachyte,"  which 
is  regarded  as  composed  essentially  of  orthoclase,  may  carry  as  much  soda  as  potash 


2O 


THE  ROMAN  COMAGMATIC  REGION. 


in  percentages,  and  that  molecularly  the  former  may  surpass  the  latter.  The  exact 
classificatory  position  of  this  type  is  clear  from  the  ratios.  As  regards  class,  order, 
and  rang,  it  falls  well  within  the  borders  of  persalane,  canadare,  and  nordmarkase; 
but  the  ratio  of  the  alkalis  shows  that  the  rock  is  transitional  between  the  sodipo- 
tassic  subrang  phlegrose,  in  which  it  actually  falls,  and  the  dosodic  subrang  nord- 
markose.  This  type  must  therefore  be  called  a  nordmarkose-phlegrose. 

Chemical  Composition  of  Ischial  Nordmarkose-phlegrose  [Augite-trachyte]. 


I. 

II. 

I. 

II. 

SiO2  

61.88     1.031 

61.4.0 

TiO2  

O  .  60       O    OOQ 

A12O3  

18.06       .178 

20.02 

ZrOj  

0.08 

Fe3O3  

2.10          .OI4 

•2.  II 

P2O,.  . 

O.O7           .OOI 

O.O2 

FeO  

1.78          .OIO 

2  .  72 

SO3  

O.  O1 

MgO 

0.61       .015 

O  .  ?2 

C12  .  . 

o  30        004 

CaO  

1  .  15           .  O2I 

1.88 

MnO    

n.d. 

Na2O  

6.89          .III 

7.  70 

BaO  

0.08 

O.OI 

Ko 

H3O+  

7-  J3 

100.52 

100.75 

H2O-  

5      °-37 

0.46 

O  =  C1  

0.07 

CO2  

none 

100.45 

I.  Ischial     phlegrose     (augite-trachyte).     Marecocco,     Ischia.     Washington,    analyst.     Am. 

Jour.  Sci.,  VIII,  1899,  p.  289. 

II.  Ischial    phlegrose    (augite-trachyte).     Marecocco,    Ischia.     Fuchs,    analyst.     Tsch.    Min. 
Mitth.,  1872,  p.  229. 


Or  

Norm  of  I. 

.  .  •JQ.  48  ) 

Ab  

.46.63  \ 

HO.  ii 

Ne  

e.ii  ) 

. 

HI*  

f 
O.  47  \ 

5-5*. 

Di  

.     3-  24  ) 

Wo  

3   "*  I 

o  .  46  S 

3-7° 

Mt  

+    < 

...     2  .  32    1 

11  

/ 

.    I  .  37  r 

4-  33 

Hm  

.    O  .  64  ) 

Ap.  . 

.    O.  17 

o.  17 

Rest  

99.89 

...   o.  c8 

91.69 


Class 


Ratios  of  I. 

Sal 
Fern 


Order 


Rang  . . . 
Subrang 


F 
L 

K-,O'+Na2O' 
CaO' 

K2O' 
'Na2O' 


=  11. 18 


=  15-43 


0.64 


100.47 
*Hl=halite  (Nad);  cf.  p.  15. 

The  figures  for  some  of  the  constituents  in  the  analysis  of  Fuchs  (II)  closely 
resemble  mine.  That  his  determinations  of  the  alkalis  are  not  correct  is  indicated 
by  the  calculation  of  the  norm  of  his  analysis,  which  proves  that  these  are  not 
present  in  sufficient  amount  to  satisfy  the  silica  and  alumina. 

Mode. — The  mode  of  the  specimen  analyzed  was  determined  by  Rosiwal's 
method,  and  is  given  in  the  table  below.  It  must  be  observed,  however,  that  no  sodalite 
could  be  detected  in  the  thin  sections  of  the  rock  analyzed,  and  a  test  on  the  section 
by  Lemberg's  method  with  dilute  nitric  acid  and  silver  nitrate  solution  gave  a  nega- 
tive result.  It  is  possible  that  it  was  present  in  crevices  of  the  piece  analyzed,  or 
else  possibly  existent  as  a  glass,  in  which  form  it  would  be  very  difficult  to  detect 


PETROGRAPHY. 


21 


among  the  fine  feldspar  laths  of  the  groundmass.  As  the  figure  for  chlorine  is  con- 
cordant with  that  shown  in  other  analyses  of  closely  related  rocks  from  the  same 
district,  it  is  justifiable  to  assume  that  the  amount  is  correct.  Furthermore,  a 
study  of  the  norm  shows  that  a  lenad  must  be  present  in  the  mode,  since  there  is  not 
enough  silica  to  form  albite  with  all  the  soda.  The  amount  of  nephelite  shown  in 
the  norm  is  just  enough  to  form  sodalite  with  the  normative  halite  (3.88  per  cent), 
leaving  only  1.70  of  nephelite  free.  Consequently,  in  the  calculated  mode  given 
in  the  first  column,  obtained  by  readjustment  from  the  norm,  the  sodalite  is  stated 
to  be  present  as  such,  while  in  the  measured  mode  it  is  bracketed  with  the  ortho- 
clase.  In  any  case  the  amount  of  this  mineral,  as  well  as  of  lavenite,  is  quite 
negligible;  so  the  mode  is  a  normative  one,  and  the  rock  may  be  described  as 
normative  trachiphyro-phlegrose. 


CALCULATED. 

MEASURED. 

Soda-orthoclase,  Or,,  Abg  

9 
86 

)          3 
4 

j 

9 

4 

Vol 

94 

2 
O 
2 

% 

16 

•25 
68 

9* 

Sp.  gr. 
X    2.6 

X  3-3 
X  3-5 
X  5-2 

=  244 

=       7 

=           2 

-     JS 

82 

43 
45 
13 

Wt 
90 

2 

O 

5 

% 

73 

75 
9i 
61 

Augite    

Lavenite    

Magnetite  

IOO 

o 

IOO 

00 

269 

83 

IOO 

oo 

Occurrence. — Ischial  phlegrose  is  especially  abundant  on  the  island  of  Ischia, 
where  it  was  found,  among  other  localities,  at  Marecocco  and  Zale,  probably  the 
site  of  the  eruption  of  about  470  B.  c.  ;*  at  Monte  Rotaro,  probably  due  to  an  erup- 
tion between  400  and  352  B.  c.;  at  Carbone  and  near  Scanella.  The  flow  at  Monte 
Olibano  in  the  Phlegrean  Fields  may  also  be  referred  to  this  type,  though  somewhat 
transitional  toward  the  cumal  type,  to  be  described  presently.  Outside  of  the 
Campanian  District  rocks  of  this  type  were  met  with  by  me  only  at  the  Molino  di 
Casa  Fredda,  in  the  Auruncan  District. 

Name. — The  name  of  the  subrang  is,  of  course,  derived  from  the  Phlegrean 
Fields,  where  this  magma  is  predominant,  and  where  specially  well-known  types 
of  it  occur.  The  type  adjective  is  derived  from  the  island  of  Ischia,  on  which  the 
type  is  abundant. 

In  the  prevailing  systems  of  classification  this  rock  is  regarded  as  a  typical 
augite-trachyte,  of  the  Ponza  typus  of  Rosenbusch,  though  with  affinities  toward 
his  phonolitic  trachyte,  through  the  presence  of  sodalite  and  lavenite. 

ISCHIAL  NORDMARKOSE-PHLEQROSE.    I.  5.  1.  4-3. 

Megascopic  characters. — Very  light  gray,  compact,  highly  porphyritic.  Feldspar  pheno- 
crysts  abundant,  10  to  20  mm.  long,  tabular  or  stout  prismatic,  clear  and  colorless.  Augite 
and  magnetite  phenocrysts  very  few,  o .  5  to  i  mm.  long ;  augite  stout  prismatic,  magnetite  equant. 
Groundmass:  very  light  gray,  fine  grained,  phanerocrystalline. 

Microscopic  characters. — Holocrystalline,  megaporphyritic,  dopatic,  magnophyric.  Phen- 
ocrysts: about  33  per  cent,  soda-orthoclase,  augite,  magnetite.  Groundmass:  about  67  per 

*  C.  W.  C.  Fuchs,  18,  p.  237. 


22  THE  ROMAN  COMAGMATIC  REGION. 

cent,  trachytic  fabric,  soda-orthoclase,  augite,  lavenite,  sodalite,  magnetite.  A  very  little  glass 
base  may  be  present. 

Soda-orthoclase. — Phenocrysts:  about  30  per  cent,  10  to  20  mm.,  euhedral  to  subhedral, 
often  fragmentary,  mostly  tabular  parallel  to  b  (oio)  or  stout  prismatic  parallel  to  axis  a, 
microperthite  and  microcline  structures  generally  wanting,  Carlsbad  twinning  common, 
inclusions  rare.  Groundmass:  about  60  per  cent,  0.05  to  0.20  mm.,  subhedral,  prismatic 
parallel  to  axis  a,  arrangement  parallel  or  subparallel. 

Augite. — Phenocrysts:  about  i  per  cent,  o.  2  to  i . o  mm.,  subhedral  to  anhedral,  prismoidal, 
very  pale  gray  or  slightly  yellowish  green,  non-pleochroic,  no  inclusions.  Groundmass:  about 
2  per  cent,  o .  05  to  o .  20  mm.,  anhedral,  prismatic,  colorless  or  very  pale  gray. 

Magnetite. — Phenocrysts:  about  i  per  cent,  0.2  to  0.4  mm.,  anhedral,  equant.  Ground- 
mass:  about  2  per  cent,  o.oi  to  0.05  mm.,  subhedral,  equant. 

Sodalite. — A  small  amount,  about  3  per  cent,  may  be  present,  either  as  equant  anhedra 
in  the  groundmass  or  in  crevices  of  the  rock,  but  is  not  essential  to  the  type. 

Lavenite. — Groundmass:  about  i  per  cent,  o.  i  to  0.2  mm.,  subhedral  prismatic  or  anhe- 
dral equant,  pale  to  deep  yellow,  nonpleochroic,  not  essential  to  type. 

Chemical  composition  as  in  analysis  I,  p.  20. 

Type  specimen  from  Marecocco,  Ischia. 

I.  5.  1.  3.   Cumal  Phlegrose  [Phonolitic  Trachyte,  Cuma  Type]. 

Megascopic  characters. — This  type  is  closely  similar  to  that  just  described,  as 
far  as  the  mode  is  concerned,  but  differs  texturally  in  the  much  smaller  number  and 
size  of  the  phenocrysts.  The  rocks  are  very  light  gray,  in  some  cases  slightly 
greenish,  and  with  a  somewhat  greasy  luster.  There  may  be  schlieric  patches  of 
darker  material,  which  does  not  differ  essentially  from  the  lighter,  except  in  color, 
but  these  are  not  regarded  as  characteristic  of  the  type. 

The  phenocrysts  constitute  only  about  10  per  cent  of  the  mass.  The  majority 
of  them  are  of  alkali-feldspar,  in  small,  subhedral  stout  prisms  or  thick  tables. 
Very  small  phenocrysts  of  sodalite  are  also  seen,  but  in  much  less  amount  than  those 
of  feldspar,  and  crystals  of  this  mineral  may  be  found  in  crevices.  Their  color  is 
sometimes  blue,  but  they  are  more  often  colorless.  Still  less  abundant  are  the 
small  prismatic  phenocrysts  of  augite. 

In  some  specimens,  as  those  of  this  type  from  the  tuffs  of  the  Vico  Volcano, 
there  are  present  rare,  rounded  white  phenocrysts  of  leucite,  about  5  mm.  in  diam- 
eter. Their  number  is  very  few,  not  over  two  or  three  being  visible  in  a  good- 
sized  hand  specimen.  The  light-gray  groundmass  is  aphanitic,  sometimes  with  a 
dull,  sometimes  with  a  slightly  greasy,  luster. 

Microscopic  characters. — In  thin  section  these  rocks  are  typically  holocrystal- 
line,  though  very  small  amounts  of  glass  may  be  present.  The  fabric  is  a  trachytic 
one,  though  in  this  type  most  of  the  specimens  do  not  show  such  a  well-marked 
fluidal  arrangement  of  the  feldspar  laths  as  in  the  preceding  one.  The  rare  pheno- 
crysts of  soda-orthoclase  offer  nothing  specially  noteworthy,  resembling  those  of  the 
ischial  type,  except  in  their  smaller  size.  The  phenocrysts  of  augite  are  either 
euhedral  or  subhedral,  are  decidedly  olive-green  in  color,  markedly  pleochroic,  and 
with  the  axis  of  greatest  elasticity  a  at  an  angle  of  30°  with  the  vertical  axis.  They 
are  therefore  of  aegirite-augite.  The  sodalites  are  either  colorless,  when  they  are 


PETROGRAPHY. 


23 


distinguishable  only  with  difficulty,  as  at  the  Phlegrean  Fields,  or  blue,  with  the 
characteristic  dusty  inclusions  arranged  in  lines,  as  in  the  blocks  from  Monte  Vico. 

The  groundmass  is  composed  in  very  large  part  of  a  soda-orthoclase  in  very 
small  prismoids,  usually  with  a  subparallel  arrangement.  There  are  also  a  few 
minute  anhedra  of  very  pale  greenish  or  grayish  augite,  occasionally  small  anhedra 
of  sodalite,  and  usually  a  little  nephelite  interstitial  between  the  feldspar  laths,  and 
often  difficult  to  detect.  Magnetite  is  usually  present  in  very  small  amount,  but 
not  invariably,  while  lavenite  is  sometimes  found.  In  some  specimens  there  are 
small  euhedral  individuals  of  titanite,  but  this  is  a  purely  accessory  constituent. 
Glass  may  be  present  in  some  cases,  but  its  amount  is  always  small,  and  it  is  difficult 
to  detect  on  account  of  the  character  of  the  groundmass. 

Chemical  composition. — Analyses  of  three  occurrences  of  cumal  phlegrose 
were  published  several  years  ago,  and  are  given  below  with  the  addition  of  deter- 
minations of  some  of  the  minor  constituents.  Those  from  the  Phlegrean  Fields 
(Monte  di  Cuma  and  Monte  Nuovo)  scarcely  differ  from  each  other,  but  the  third, 
from  Monte  Vico,  shows  a  very  considerably  greater  amount  of  potash  relatively 
to  the  soda,  though  the  ratios  of  the  alkalis  in  all  three  are  so  near  unity  as  to 
cause  all  to  fall  in  the  same  sodipotassic  subrang.  It  is  also  seen  that  in  the  Monte 
Vico  rock  sulphur  trioxide  partially  replaces  the  chlorine  of  the  other  two,  though 
the  amounts  are  so  small  in  all  cases  as  to  be  negligible  for  purposes  of  classifica- 
tion. The  analysis  by  vom  Rath  (IV)  is  very  probably  of  the  same  type  as  III,  but 
it  is  almost  certain  that  through  some  error  the  alkalis  have  been  interchanged. 

CJtentical  Composition  of  Cumal  Phlegrose  [Phonolilic  Trachyte.] 


I. 

II. 

in. 

IV. 

SiO              

CO    7O 

08? 

60.  18 

A12O3    

TO.  OC 

l8<7 

l8    9"J 

T7O 

18  67. 

182 

18.70 

Fe2O3  

2.QC 

.  OIO 

2.84 

.018 

7.  7O 

O2I 

FeO  

I.  08 

.OIC 

I    20 

018 

I  .  2O 

OI7 

3.44 

MgO  

o.  3.6 

.OOQ 

0.38 

.  OIO 

O.  12 

OO2, 

O.  7,2 

CaO  

1  .  10 

.O2I 

X.K 

.021 

2.o6 

O7.7 

2.80 

Na2O  

6.  7O 

.  I  IO 

7    1C 

IIC 

4.87 

O7O 

0.6? 

K2O  

7.  10 

.076 

7.  3O 

.078 

0.  14 

OQ7 

4.18 

H2O  +  

O.  2  A. 

o  c6 

0.86 

CO2  

none 

none 

none 

\    °-33 

TiO2  

o.  c6 

.OO7 

0.47. 

.ooc 

o.  c6 

OO7 

ZrO2  

O.  IO 

OOI 

P2Or     . 

o.  10 

.OOI 

O    OA 

O.  1C 

OOI 

SO,  .  .  . 

none 

none 

o.  17 

OO2 

0.19 

Cl  

O.  C7 

.  OI4 

O  .  42 

OI2 

0.08 

OO2 

o.  14 

MnO  

n.d. 

n.d. 

n.d. 

BaO  

0.06 

O  =  C1  

99-74 

O.  12 

100.  17 

O.  IO 

100.54 

O.O2 

99-95 

99.62 

100  07 

100.52 

Sp.  gr.  .  . 

2.  COO 

THE  ROMAN  COMAGMATIC  REGION. 


NOTES  EXPLAINING  HEADINGS  OF  PRECEDING  TABLE. 

I.  Cumal  nordmarkose-phlegrose  [phonolitic  trachyte].     Monte  di  Cuma,  Phlegrean  Fields. 

Washington,  analyst.     Cf.  Am.  Jour.  Sci.,  VIII,  1899,  287. 
II.  Cumal  nordmarkose-phlegrose  fphonolitic  trachyte].      Monte  Nuovo,   Phlegrean  Fields. 
Washington,  analyst.     Cf.  Am.  Jour  Sci.,  VIII,  1899,  287. 

III.  Cumal  phlegrose  [phonolitic  trachyte].     Block  in  tuff,  Via  Aurelia,  4  km.  SW.  of  Viterbo, 

Ciminian  District.     Washington,  analyst.     Cf.  Jour.  Geol.,  IV,  1896,  849. 

IV.  Cumal  phlegrose  [phonolitic  trachyte].     Monte  Cimino,  near  Viterbo.     vom  Rath,  analyst. 

Zeits.  d.  d.  geol.  Ges.,  XVIII,  1866,  p.  581,  alkalis  interchanged  ? 


Norms. 


Or  

I. 

.42.  26  ^ 

II. 

43-37' 
35-37     78-74 

0  .  OO 

O.  70    •  IO.2I 
O.OO 

2.91 
o.oo  •    7.54 

7.08     3-70 

'             2.32 
0.76  •  3.08 

o.oo 
o.oo 

Ab  

.  .  TO.  3XD  >  8l.  78 

An  

.     2.  22    J 

Ne  

.    7   ne  ^ 

HI*  

.  .  .    0.82  >    8.77 

Th*  

.    O.OO  ) 

Di  

< 

I  .  04  ) 

Wo  

.   o  .  46  r    2  .  40 

Ac  

o  oo  } 

Mt  

i  .  86  ) 

11  

i  .  06  f   4  .  68 

Hm  

.   i  76  ) 

Ap  

o  .  oo 

Rest  

99-63 

0.34 

99-57 
0.60 

99-97 
Class  

100.17 

Ratios. 
I. 
Sal 
Fim-                 =I3'°7 
F 
L 
K2O'+Na2O' 

Order  

CaO' 
K20' 

Na20' 

•95 


10.62 


II. 

8.38 


53-93 

26.78 

2.78 

6.96 

0.  12 
0.28 
0.65 

2-44 
o.oo 

2.32 

l'.76 
Q-34 

99.62 

1.  02 

100.64 


III. 
10.62 


III. 


83-49 


3-°9 


5.14 


91.05 


7.71    11.37 


o-73 


1.23 


*For  HI  and  Th  see  p.  15. 


From  these  norms  and  ratios  it  is  seen  that  the  rocks  from  the  Phlegrean 
Fields  approach  the  border  of  the  dosodic  subrang  nordmarkose,  like  the  ischial 
type,  and  they  might  be  considered  as  transitional  and  the  subrang  be  called  nord- 
markose-phlegrose. On  the  other  hand,  the  Vico  rock  is  close  to  the  center  of 
the  sodipotassic  subrang,  and  the  potash  indeed  slightly  surpasses  the  soda.  In 
other  respects  they  are  all  well  within  the  limits  of  class,  order,  and  rang. 

Modes. — The  mode  of  the  Cuma  specimen,  both  as  calculated  from  the  norm 
and  as  measured  by  Rosiwal's  method,  is  given  below.  In  the  former  the  amount 
of  sodalite  was  calculated  on  the  basis  of  the  figures  for  chlorine  reported,  and  is 
seen  to  be  very  close  to  the  measured  value.  As  the  composition  of  the  aegirite- 
augite  was  unknown,  and  the  whole  of  the  normative  hematite  would  yield  far  too 
much  aegirite,  an  amount  of  Na2O  (.004)  equivalent  to  that  of  CaO  in  the  wol- 


PETROGRAPHY. 


25 


lastonite  was  taken  as  a  basis,  which  would  give  a  composition  approximately  like 
that  of  most  aegirite-augites,  the  rest  of  the  hematite  being  placed  with  the  ores. 
The  CaO  of  the  wollastonite  takes  up,  then,  just  the  amount  of  A12O3  set  free  from 
nephelite  by  the  transfer  of  Na2O  to  acmite,  to  form  anorthite  (3.3  per  cent)  which, 
with  that  calculated  normatively,  must  exist  in  the  feldspar.  A  small  amount  of 
nephelite  is  necessarily  present,  but  this  would  easily  escape  observation,  owing  to 


CALCULATED. 


MEASURED. 


Soda-orthoclase, 

Nephelite 

Sodalite 

jEgirite-augite  .  . 
Ores 


:1J 


84 
3-9 
4-° 

100.  O 


Vol.  %       Sp.gr. 
84.7    X    2.6 

4-9     X     2.2     < 

6.6   X  3-3 
3-8   X  5.2 


230.2 
10.8 

21.8 


Wt.% 
8i-5 

3-8 

7-7 
7.0 


282.6     100.0 


the  fabric  of  the  groundmass.  Allowing  for  these  readjustments,  the  average 
composition  of  the  alkali-feldspar  would  be  Or76Ab7S;  that  is,  almost  exactly 
OrxAb,. 

In  the  measured  mode  the  nephelite  has  been  measured  with  the  feldspar,  and 
the  amount  of  these  is  seen  to  be  rather  less  than  the  calculated  amount.  On  the 
other  hand,  the  measured  amounts  of  pyroxene  and  ores,  the  colored  minerals,  are 
rather  higher  than  those  calculated  by  the  same  figure,  these  for  sodalite  being 
almost  identical.  This  discrepancy,  though  not  serious,  may  be  attributed  to  the 
overlapping  of  the  small  mineral  grains,  and  a  consequent  greater  apparent  amount 
of  the  colored  ones,  a  point  we  shall  have  to  notice  later. 

The  mode  of  the  blocks  from  Monte  Vico  was  also  calculated  and  measured 
with  the  following  results.  The  process  of  calculation  was  very  similar  to  the  pre- 


CALCULATED. 


MEASURED. 


Orthoclase,  Or2Abr . 

Nephelite 

Haiiyne 

^Egirite-augite 

Ores 

Titanite . . 


82 


Vol.  %       Sp.  gr. 


Wt.% 


.7 
5-4 

2.1 

4-5 
4.2 
0.6 


85.3   X   2.6   =   221.8      81.4 


2.9 

8.6 

2.7 


X  2.3 
X  3-3 
X  5-2 
X 


3-5   = 


99-5 


6.7 

28.4 

14.0 

1.7 
272.6     100.0 


2.4 

10.4 

5.2 

0.6 


ceding.  The  measured  amount  of  titanite  was  assumed  to  be  correct,  and  the  FeO 
thus  set  free  from  normative  ilmenite  placed  in  a  diopside  molecule,  the  extra  lime 
being  taken  from  normative  wollastonite.  A  small  amount  of  zegirite  was  arbi- 
trarily calculated,  as  in  the  last  case,  and  an  amount  of  anorthite  corresponding  to 
this  calculated,  the  total  amount  of  this  being  3.6,  which  goes  into  the  feldspar. 


26  THE  ROMAN  COMAGMATIC  REGION. 

The  wollastonite  which  was  left  over  was  placed  in  the  aegirite-augite  molecule 
though  it  is  a  trifle  high.  There  not  being  enough  SiO2  left  to  form  albite  from  all 
the  Na2O  present,  nephelite  was  calculated  by  the  same  set  of  equations  used  to  cal- 
culate the  norm,  exactly  as  was  done  in  the  preceding  mode.  The  average  alkali- 
feldspar,  then,  will  be  Or97Ab48,  or  almost  exactly  Or2Abx. 

Comparison  with  the  measured  mode  shows  the  same  relations  as  in  the  case 
of  the  Cuma  rock,  though  here  it  is  considerably  greater,  and  is  also  to  be  attrib- 
uted to  overlapping.  The  calculated  and  measured  amounts  of  haiiyne  are  very 
concordant,  which  in  both  cases  is  to  be  attributed  to  the  rather  fair  size  of  the 
crystals,  equaling  or  excelling  the  thickness  of  the  section,  and  their  equant  shape 
and  consequent  small  liability  to  overlapping. 

The  mode  of  the  Monte  Nuovo  rock  was  not  calculated  or  measured,  as  it  was 
deemed  to  be  superfluous.  It  may  be  assumed  to  correspond  closely  with  that  of 
the  Cuma  phlegrose. 

Taking  all  the  above  facts  into  consideration,  we  may  conclude  that  the  mode 
of  cumal  phlegrose  is  best  represented  by  the  two  calculated  modes  given  above, 
rather  than  by  the  measured  ones.  The  variations  from  the  norm  are  inconsiderable, 
and  these  rocks  can  readily  be  classified  as  far  as  rang  by  a  microscopic  examina- 
tion alone,  and,  if  the  highly  sodic  character  of  the  alkali-feldspar  were  known,  as 
far  as  subrang.  They  would  therefore  be  described  as  normative  trachiphyro- 
phlegrose. 

Occurrence. — Cumal  phlegrose  is  most  typically  and  abundantly  represented  in 
the  Phlegrean  Fields,  where  it  occurs  at  Monte  di  Cuma,  Monte  di  Procida,  and  as 
blocks  at  Monte  Nuovo,  as  well  as  other  localities.  The  well-known  "piperno"  of 
Pianura  might  be  considered  to  be  of  this  type,  though  the  entire  absence  of  pheno- 
crysts  and  the  peculiar  eutaxitic  texture  shown  in  the  "flames"  of  darker  material 
would  justify  the  establishment  of  a  separate  type  for  this,  which  might  be  called 
the  pianural.  The  cumal  type  is  also  frequent  on  Ischia,  as  at  Monte  Tabor, 
Monte  Rotaro,  Punto  di  Castiglione,  Scanella,  Panza,  and  elsewhere.  Outside  of 
the  Campanian  District  these  rocks  are  comparatively  rare.  They  are  met  with  in 
the  Auruncan  District,  where  they  form  flows  east  of  Casi  near  Teano,  below  Orchi, 
and  possibly  elsewhere,  and  in  the  Ciminian  District,  where  the  type  occurs  as 
blocks  in  yellow  tuffs  around  Monte  Vico  and  possibly  as  a  few  flows. 

Name. — The  type  name  is  derived  from  the  locality  of  Cuma  in  the  Phlegrean 
Fields,  the  earliest  Greek  settlement  of  the  neighborhood  and  a  prominent  locality 
for  the  type. 

In  the  prevailing  classifications  these  rocks  would  be  properly  called  trachyte, 
and  would,  for  the  most  part,  fall  under  the  head  of  phonolitic  trachytes  of  Rosen- 
busch,  though  he  classes  some  of  the  occurrences  as  acmite-trachyte,  and  others  as 
augite-trachyte.  The  rocks  of  this  type  from  Monte  Vico  were  formerly  called 
phonolite  by  me,  but  this  name  lay  undue  stress  on  the  presence  of  nephelite  and 
haiiyne,  the  amounts  of  which  are  in  reality  very  small. 


PETROGRAPHY.  27 

CUMAL  PHLEdROSE.    I.  3.  1.  3. 

Megascopic  characters. — Light  gray,  compact,  slightly  porphyritic.  Feldspar  phenocrysts 
rare,  i  to  5  mm.,  stout  prismatic  or  tabular,  clear  and  colorless.  Sodalite  or  haiiyne  pheno- 
crysts rarer,  not  always  visible,  about  i  mm.,  colorless  or  blue,  equant.  Augite  phenocrysts 
very  rare,  about  i  mm.,  prismatic,  black.  Groundmass:  aphanitic,  light  gray,  usually  dull, 
but  sometimes  with  slightly  greasy  or  silky  luster,  often  mottled  with  darker  schlieren. 

Microscopic  characters. — Holocrystalline,  megaporphyritic,  perpatic,  mediophyric.  Phen- 
ocrysts: 10  per  cent  or  less,  soda-orthoclase,  sodalite  or  haiiyne,  aegirite-augite.  Groundmass: 
90  per  cent  or  more,  soda-orthoclase,  sodalite  or  haiiyne,  nephelite,  aegirite-augite,  magnetite, 
titanite,  occasionally  a  little  glass. 

Soda-orthoclase,  Or2Abj  to  O^Ab,. — Phenocrysts:  about  5  per  cent,  i  to  5  mm.,  euhe- 
ral  to  subhedral,  tabular  parallel  to  b  (oio)  or  stout  prismatic,  microperthite  and  microcline 
structures  wanting,  Carlsbad  twinning  common,  inclusions  rare.  Groundmass:  about  90  per 
cent,  0.05  to  0.20  mm.,  subhedral  prismatic,  arrangement  parallel  or  subparallel. 

Sodalite  or  haiiyne. — Phenocrysts:  about  3  per  cent,  0.5  to  2.0  mm.,  subhedral,  equant, 
colorless  or  blue,  inclusions  sometimes  common,  dusty,  in  fine  lines.  Groundmass:  about  2 
per  cent,  same  as  phenocrysts  but  much  smaller. 

Mgirite-augite. — Phenocrysts:  about  2  per  cent,  0.5  to  2.0  mm.,  subhedral,  prismatic, 
pale  olive-green,  pleochroic.  Groundmass:  about  2  per  cent,  0.05  to  o.  10  mm.,  subhedral,  pris- 
matic, pale  green  or  gray. 

Magnetite. — Groundmass:  about  3  per  cent,  0.05  to  0.5  mm.,  anhedral,  equant. 

Titanite. — Groundmass:  about  0.5  per  cent,  0.5  mm.,  euhedral. 

Nephelite. — Groundmass:  about  5  per  cent,  formless  areas,  interstitial  between  the  feld- 
spar prisms,  colorless. 

Chemical  composition  as  in  analyses  I,  II,  and  III,  p.  23. 

Type  specimens  from  Monte  di  Cuma  and  Monte  Nuovo,  Phlegrean  Fields,  and  Monte 
Vico,  Ciminian  District. 

I.  5.  1.  3.    Rotaral  Phlegrose  [Trachyte  Obsidian,  Rotaro  Type]. 

Megascopic  characters. — Megascopically  these  rocks  show  rather  numerous 
(about  15  per  cent)  white  phenocrysts  of  alkali-feldspar,  in  stout  prisms,  from 
i  to  5  mm.  long,  and  which  stand  out  prominently  against  the  dark  groundmass. 
The  very  dark  gray  or  black  groundmass  is  either  obviously  hyaline  to  the  naked  eye 
or  else  aphanitic  and  with  a  rough  fracture;  but  even  in  this  last  case  examination 
with  the  lens  and  the  natural  surfaces  of  the  lava  blocks  shows  that  it  is  highly  vitreous. 

Microscopic  characters. — In  thin  section  the  feldspar  phenocrysts  are  clearly 
of  a  sodic  orthoclase,  usually  with  Carlsbad  twinning,  and  the  few  small  phenocrysts 
of  augite  show  a  very  pale  gray  or  yellowish  green  and  those  of  biotite  a  brown 
color.  The  groundmass  varies  considerably,  enough  so  as  to  permit  the  establish- 
ing of  several  distinct  types,  were  this  desirable.  In  some  cases,  as  in  specimens 
from  the  Monte  di  Procida,  as  well  as  some  Ischian  localities,  it  is  an  almost  pure 
glass  of  a  light-brown  color  and  quite  free  from  microphenocrysts  or  microlites. 
In  others,  as  in  the  Monte  Rotaro  rocks  most  typically,  it  is  quite  thickly  strewn 
with  small,  prismatic  microphenocrysts  of  sodic  feldspar,  which  are  sometimes  much 
branched.  It  was  noticed  that  the  rocks  in  which  the  glass  cement  is  most  free 
from  these  small  crystals  are  very  friable  and  break  into  small  pieces  under  the 
hammer,  not  showing  the  solidity  and  coherence  of  such  well-known  obsidians  as 
those  of  Lipari,  Iceland,  or  the  Yellowstone  Park. 


28 


THE  ROMAN  COMAGMATIC  REGION. 


Chemical  composition  and  mode. — Only  one  analysis  was  made  of  this  type,  and 
this  of  a  rock  of  somewhat  exceptional  textural  character.  While,  therefore,  in  the 
absence  of  analysis,  and  in  view  of  the  abundance  of  the  glass  base  and  the  conse- 
quent indeterminability  of  the  mode,  the  magmatic  position  can  not  be  definitely 
established  for  all  the  specimens,  there  is  little  reason  to  doubt  that  all  those 
obtained  in  the  Campanian  District  belong  to  the  subrang  phlegrose,  where  the 
great  majority  of  the  holocrystalline,  and  hence  determinable,  rocks  fall.  There 
is  all  the  more  reason  for  this  belief  in  the  fact  that  no  soda-lime  feldspars  were 
observed  either  among  the  phenocrysts  or  the  microlites.  In  the  rocks  of  the  Phle- 
grean  Fields  which  fall  in  vulsinose,  and  which  carry  modal  labradorite,  this  mineral 
is  invariably  present  as  large  phenocrysts.  It  is  therefore  justifiable  to  think  that, 
were  these  rocks  in  question  domalkalic,  they  would  show  phenocrysts  of  labradorite. 
No  definite  proof  can  be  given  to  show  that  they  are  sodipotassic,  except  that  of 
an  analysis,  and  it  is  possible  that  some  of  them  are  dosodic,  and  hence  in  nord- 
markose.  But  when  the  general  character  of  the  district  is  considered,  this  does 
not  seem  very  probable. 

Chemical  Composition  of  Rotaral  Phlegrose  {Trachyte  Obsidian}. 


I. 

II. 

I. 

II. 

SiO 

61.62     1.027 
i8.ii     0.177 
2.36       .015 
1.28       .018 
0.56       .014 
i  .  44       .  026 

5-77       -093 
7.60       .081 
0.78 
0.87       .on 
0.13       .001 

60.77 

19.83 

4.14 
2.43 

o-34 
1.63 
4.90 
6.27 
0.46 
n.d. 
trace 

Cl  

0.15     0.004 
n.d. 

trace 

Al  O, 

MnO  

TT«   (~\ 

Or*! 

rCjUj  
FeO  

100.67 
0.03 

100.55 

MeO.  . 

CaO  

Na2O    

Sp.gr  

K2O  .          

100.64 

HO-U 

TiOj 

2-44 

P,Oc. 

I.   Rotaral  phlegrose  [trachyte-obsidian].      Monte  Rotaro,  Ischia.     Washington,  analyst.  Cf 

Am.  Jour.  Sci.,  VIII,  1899,  289. 

II.   Rotaral  phlegrose  [trachyte-obsidian].     Monte  Rotaro,  Ischia.     Fuchs,  analyst.    Tsch.  mm. 
Mitth.,  1872.     p.  232. 


Or         

Norm 

of-I. 

04 

39       8.82} 
39                f 

40      3-63' 
23 

3  3.49 
62) 

67^   4-57 
28) 
24      o-34 

Ab  

41  . 

An  

i  . 

Ne  

2 

HI    . 

Di  

, 

Wo  

Mt       

i 

11  

i 

Hm  

i 

Ap.. 

'O 

Class , 


91-45 


8.40 


Order. 


Ratios  of  I. 

Sal 

Fern 

F 
L 


Rang..., 
Subrang. 


KgQ'+Na,O' 
CaO' 


99-85 

Rest 0.78 

100.63 


Na,O' 


=24.19 
34.80 
=  0.87 


PETROGRAPHY.  29 

The  very  close  resemblance  between  this  analysis  and  the  preceding  ones  is 
evident,  though  the  norm  shows  that  this  rock  is  nearer  all  the  center  points  of 
phlegrose  than  any  of  the  others.  This  is  especially  true  of  the  ratio  of  soda  to 
potash,  which  is  very  close  to  unity,  instead  of  approaching  the  dosodic  border  as 
in  the  other  phlegroses  of  the  district. 

As  the  mode  is  indeterminate  through  the  presence  of  abundant  glass,  it  is  not 
possible  to  discuss  the  relations  of  norm  and  mode.  It  may  be  observed,  however, 
that  the  greater  part  of  the  normative  augite  and  magnetite  molecules,  with  the 
sodalite,  have  not  crystallized  out,  but  exist  in  the  glass  base.  Were  the  rock  holo- 
crystalline,  its  mode  would  have  been  undoubtedly  almost  identical  with  that  of  the 
Cuma  phlegrose,  and  the  average  feldspars  would  have  had  the  composition 
OrjAbx. 

Occurrence. — In  the  Roman  Region  the  rotaral  type  is  found  only  in  the  Cam- 
panian  District,  and  even  here  is  by  no  means  abundant.  Among  localities  which 
may  be  mentioned  are  Monte  di  Cuma  and  Monte  di  Procida  in  the  Phlegrean 
Fields,  and  Monte  Rotaro  and  Scanella  on  Ischia.  It  is  noteworthy  that  this  highly 
vitreous  texture  is  never  assumed  by  the  same  magma  in  the  other  districts  where 
it  occurs,  the  Auruncan  and  the  Ciminian,  where  the  phlegrose  rocks  are  uniformly 
holocrystalline. 

Name. — The  type  name  is  derived  from  Monte  Rotaro,  the  locality  of  the  type 
specimen.  In  prevailing  classifications  this  type  would,  of  course,  be  considered  a 
trachyte-obsidian. 

ROTARAL  PHLEGROSE.    I.  5.  1.  3. 

Megascopic  characters. — Compact,  black  sprinkled  with  white  crystals,  obviously  highly 
vitreous,  porphyritic.  Phenocrysts  of  feldspar  rather  abundant,  i  to  5  mm.,  stout  prisms,  white 
and  colorless,  prominent.  Groundmass  black  or  brownish  black,  highly  vitreous,  often  with 
conchoidal  fracture,  sometimes  rough. 

Specific  gravity,  2.44. 

Microscopic  characters. — Dohyaline,  mega-  and  micro-porphyritic,  dopatic,  mediophyric. 
Phenocrysts,  about  15  per  cent,  feldspar.  Groundmass:  about  85  per  cent,  mostly  glass,  with 
some  microphenocrysts  of  alkali -feldspar,  augite  and  biotite. 

Soda-orthodase. — Megaphenocrysts :  about  15  per  cent;  i  to  5  mm.;  subhedral  to  euhedral, 
stout  prismatic;  Carlsbad  twinning  rare,  inclusions  rare.  Microphenocrysts  very  variable, 
sometimes  about  50  per  cent,  sometimes  almost  none;  o.  10  to  0.30  mm. ;  subhedral,  prismatic, 
often  branched  in  sheaflike  forms;  arrangement  often  subparallel. 

Augite. — Microphenocrysts:  about  2  per  cent;  o.i  to  0.2  mm.;  euhedral,  prismatic; 
pale  gray  or  very  pale  greenish. 

Biotite. — Microphenocrysts:  about  i  per  cent,  not  always  present;  0.05  to  0.02  mm.;  thick 
tabular;  brown,  unaltered. 

Glass. — About  80  per  cent;  usually  brown,  sometimes  almost  colorless.  Sometimes  dusty 
with  minute,  indeterminate  microlites. 

Chemical  composition  and  norm  as  on  p.  28. 

Type  specimen  from  Monte  Rotaro,  Ischia. 


30  THE  ROMAN  COMAGMATIC  REGION. 

I.  5.  2.  2.    Bolsenal  Vulsinose  [Vulsinite,  Bolsena  Type]. 

As  is  stated  elsewhere,  this  type  is  homologous  texturally  with  the  cumal  type 
of  phlegrose,  differing  from  the  arsal  type  just  as  the  cumal  differs  from  the 
ischial.  Modally,  on  the  other  hand,  the  arsal  and  bolsenal  vulsinose  differ 
from  ischial  and  cumal  phlegrose  by  the  presence  of  considerable  anorthite  or 
labradorite. 

Megascopic  characters. — These  rocks  are  light  gray,  in  specimens  from  one 
locality  somewhat  mottled  with  streaks  of  darker  gray  and  yellowish.  As  the 
"schlieren"  seem  to  be  due  rather  to  slight  weathering  than  to  essential  differences 
in  composition,  they  are  not  of  sufficient  importance  to  justify  the  separation  of 
these  specimens  from  the  others  as  a  distinct  type.  While  the  type  is  distinctly 
porphyritic,  it  is  not  conspicuously  so,  as  in  the  preceding  one.  The  phenocrysts, 
which  are  mostly  of  feldspar  with  fewer  of  augite  and  biotite,  are  small,  from  i  to  5 
mm.  long,  and  not  very  abundant,  making  up  not  more  than  one-fifth  of  the  rock 
volume.  The  feldspar  and  biotite  phenocrysts  are  tabular,  as  usual,  while  those  of 
augite  are  stoutly  prismatic.  The  light-gray  groundmass  is  aphanitic. 

Microscopic  characters. — The  tabular  phenocrysts  of  feldspar  are  seen  in  thin 
section  to  be  of  both  orthoclase  and  anorthite,  the  former  more  abundant  than  the 
latter.  They  are  both  subhedral,  with  some  crystal  planes,  occasionally  fragmen- 
tary. Carlsbad  twinning  is  common  in  the  former  feldspar,  while  the  anorthite  is 
often  quite  untwinned,  though  usually  with  multiple  lamellae.  Zonal  structure  is 
not  often  seen,  and  inclusions  are  not  very  common,  of  augite,  magnetite,  apatite, 
and  glass  in  both,  and  also  of  anorthite  in  the  orthoclase.  In  many  specimens  the 
phenocrysts  of  both  feldspars  are  surrounded  by  a  border  of  alkali- feldspar  sub- 
stance of  later  growth,  which  is  oriented  parallel  crystallographically  with  the 
inclosed  crystal,  extinguishes  simultaneously  all  around  it,  and  extends  a  short  dis- 
tance into  the  groundmass,  without  definite  form.  The  augite  phenocrysts  call  for 
little  notice.  They  are  mostly  subhedral,  in  stout  prismoids,  of  the  usual  pale-gray 
color  so  characteristic  of  the  rocks  of  the  region,  and  contain  few  inclusions  of  mag- 
netite and  of  glass.  The  biotite  phenocrysts  are  tabular,  with  irregular  outlines,  of 
a  pale  brown,  and  are  uniformly  much  altered  in  the  usual  way.  A  few  phenocrysts 
of  magnetite,  in  irregular  grains,  may  generally  be  seen. 

The  groundmass  is  holocrystalline  and  composed  in  great  part  of  small  pris- 
moids of  soda-orthoclase,  which  frequently  have  a  subparallel  arrangement,  giving 
rise  to  a  trachytic  fabric.  With  them  are  some  very  small  prismoids  of  colorless 
augite  and  still  fewer  small  anhedra  of  magnetite.  A  little  titanite  and  apatite  are 
seen  in  accessory  amounts,  and  some  specimens  show  rare  flakes  and  irregular  areas 
of  a  pale-brownish  hornblende;  but  none  of  these  minerals  can  be  considered  as 
essential  to  the  type. 


PETROGRAPHY. 


Chemical  composition. — Analyses  of  bolsenal  vulsinose  are  available  from  two 
occurrences.  Those  of  the  Bolsena  rock  were  made  on  material  from  the  same  flow 
but  of  different  specimens.  One  has  already  been  published,  but  another  analysis 
was  made,  since  the  former  was  only  the  second  complete  rock  analysis  which  I 
had  made,  and  a  test  of  my  work  at  that  time  was  desirable.  The  results  are  satis- 
factorily concordant,  especially  when  it  is  considered  that  they  were  made  on  dif- 
ferent specimens  of  a  somewhat  eutaxitic  lava.  As  none  of  the  sample  of  powder 
from  the  first  analysis  was  preserved,  the  figures  for  TiO2  and  P2OS  given  in  II  have 
been  inserted  in  I,  with  proper  corrections  in  the  amount  of  A13O3.  This  is  not 
strictly  accurate,  but  as  both  specimens  come  from  the  same  flow,  and  the  figures 
in  question  are  small,  the  error  can  not  be  of  importance,  and  the  analysis  as 
thus  corrected  and  completed  is  undoubtedly  more  nearly  in  accordance  with  the 
facts  than  as  it  was  originally  published.  The  two  analyses  of  the  Astroni  rocks  by 
Riva  have  been  used,  as  they  show  evidence  of  accuracy  by  correspondence  with 
the  descriptions  and  figures  of  the  mode,  and  are  satisfactorily  complete. 

Chemical  Composition  of  Bolsenal  Vulsinose  [Vulsinite]. 


I. 

II. 

III. 

IV. 

V. 

SiO3  

(8.21       O.O7O 

e7.nr      0.066 

58.08     0.968 

57.  ?8     0.060 

57.  5O     o  nc8 

A12O3  

18.88       .185 

IQ.  77         .  180 

IQ.II           .l87 

IQ.  70          .  IQO 

18.80       .  i84 

Fe,C>7 

4.  O7          .026 

3  .07         .OIO 

7  .  55           .027 

7.22          .O2O 

4.  77            028 

FeO  

0.87      .on 

I  .  12         -015 

i.oo       .014 

1.62           .022 

o  .  62        009 

MgO  

O.O8          .025 

1.  12         .028 

I  .  O5          .  O26 

I.I7          .029 

I  .  2O          .O7O 

CaO  

7.  58       .046 

7.  07         .O7O 

7  .  76          .  067 

4.o8          .077 

7  •  84        .  060 

Na2O  

2.57        .042 

3.10       .050 

2  .  84          .  046 

3-12           .050 

7.  l6          .O5I 

K2O  

0.17         .  008 

97        °9 

8.55       .001 

8.86       .091; 

8.68          .097 

8.7Q          .080 

H2O+     .... 

O.  54 

O.  54 

O.  5O 

o.  61 

H,O-  

0.74 

O.  II 

O.  II 

0.44 

0.78 

CO2  

none 

none 

none 

TiO,  

0.82       .010 

0.82       .010 

0.82       .010 

0.32        .  004 

0.50       .006 

P,Oc.. 

0.20       .001 

0.20       .001 

0.20       .001 

O.2I           .OOI 

0.28       .002 

Cl  

o.  17        .005 

o  .  03       .  ooo 

MnO  

n.d. 

n.d. 

n.d. 

0.57 

100.09 

99.80 

99.92 

100.49 

100.21 

Sp.  er.  .  . 

2  .  5  34.  at  25° 

I.  Bolsenal  vulsinose  [vulsinite].     Below  castle,  Bolsena,  Vulsinian  District.     H.  S.  Wash- 
ington, analyst.     Jour.  Geol.,  IV,  1896,  p.  552.     TiO2  and  P2O5  from  II. 
II.  Bolsenal  vulsinose  [vulsinite].     Below  castle,  Bolsena,  Vulsinian  District.     H.  S.  Wash- 
ington, analyst. 

III.  Bolsenal  vulsinose  [vulsinite].     Below  castle,  Bolsena.     Mean  of  I  and  II. 

IV.  Bolsenal  vulsinose   [vulsinite].     Caprara,  Astroni,   Phlegrean   Fields.     C.  Riva,  analyst. 

De  Lorenzo  and  Riva,  Alt.  Ace.  Nap.  Sci.,  XI,  1902,  p.  43. 

V.  Bolsenal  vulsinose  [vulsinite].     Pagliaroni,  Astroni,  Phlegrean  Fields.     C.  Riva,  analyst. 
De  Lorenzo  and  Riva,  Alt.  Ace.  Nap.  Sci.,  XI,  1902,  p.  48. 


THE  ROMAN  COMAGMATIC  REGION. 


Norms. 
III.                                                IV. 
Or    52-82                                        Si-7i 

V. 

48") 

Ab    23.06      88.67)                     21.48      86 

.8^                     25 

re  I   86 

86  j 

An                     .12.70                    (.80.24          13.62 

L    89.  02         12 

2?    ) 

Ne     .           .  .  .      o.  <57                   \                          i.  08 

\                                  o 

8c  ) 

| 

Hi        0-23 

[     o 

DJ     3-67  \                                        4-32  ) 

3 

8q  > 

Ol            0.63)      4'3°                           0.63  5      4 

84  \      4 

73 

Mt    °-93  )                                       4-  J8  ) 

o 

7O  ) 

11      1.52  >      5-49       10.19          0.61  >      5 

.11       10.46           o 

'      f 

Oi  r      "> 

6r 

Hm        3-°4  )                                       °-32  ) 

4 

v    \      • 
oo  j 

Ap          0.40        0.40                          0.40        o 

.40                           o 

67         o 

67 

99-43                                       99-48 
Rest           •••     0.65                                          °-94 

98 

i 

72 
SO 

100.08                                      100.42 

Ratios. 
11 

Class  —                  -     8 

IOO 

I.            IV.              -1 
.  76         8  CT           7 

30 

F 

Order                      .  —                         —  TCC 

K3O'+Na2O' 

•"•«*•"&  CaO' 

K30' 

Subrane  .  .          .  .  r;  —  -^r-.              =     2 

.07         2.92           3 
.07         i  .  86           i 

J9 

tfl 

Js,7I 


ii. 01 


The  very  close  correspondence  of  these  rocks,  from  near  the  extremities  of  the 
volcanic  zone,  establishes  the  type  of  bolsenal  vulsinose  with  great  definiteness.  As 
regards  class,  the  ratios  approach  somewhat  closely  to  the  border  of  dosalane,  though 
not  near  enough  to  justify  us  in  calling  the  type  transitional.  The  ordinal  position 
in  every  case  is  well  within  the  limits,  the  amount  of  normative  lenads  being  entirely 
negligible.  The  ratios  determining  rang  are  almost  exactly  at  the  center  point  in 
all  three  analyses,  but  as  regards  subrang  all  three  approach  the  border  toward  the 
third  (sodipotassic)  division.  This  is  especially  true  of  V,  which  might  be  desig- 
nated properly  a  pulaskose-vulsinose,  though  it  would  seem  to  be  a  rather  hair- 
splitting matter  to  call  it  a  separate  type  on  this  ground,  when  the  other  represen- 
tative from  the  same  volcano  resembles  it  so  closely. 

Mode. — Complete  estimation  of  the  mode  by  Rosiwal's  method  was  impos- 
sible, on  account  of  the  small  size  and  confused  arrangement  of  the  constituents  of 
the  groundmass,  as  well  as  the  error  due  to  overlapping,  although  the  phenocrysts 
could  be  measured  with  sufficient  accuracy  in  the  thin  sections.  The  figures  below 
are  therefore  based  partly  on  Rosiwal  measurements  and  partly  on  recalculation 

of  the  norm. 

Soda-orthoclase,  Or9Ab4 69 . 5 

Anorthite 6.1 

Andesine-labradorite,  Abj Anr 11.9 

Augite 7.4 

Biotite i .  o 

Ores 3.6 

Apatite 0.4 

Titanite o.  i 


100.0 


PETROGRAPHY.  33 

It  is  clear  from  this  that  considerable  soda-lime  feldspar  must  exist  among  the 
groundmass  feldspar  laths,  which  is  indistinguishable  from  those  of  orthoclase,  partly 
due  to  their  small  size  and  partly  to  the  absence  of  twinning,  which  is  not  always 
pronounced  even  in  the  phenocrysts  of  anorthite.  That  the  groundmass  soda-lime 
feldspar  must  be  considerably  more  sodic  than  the  phenocrysts  is  shown  by  their 
lower  refractive  index,  as  well  as  from  a  consideration  of  the  norm  and  the  composi- 
tion of  the  orthoclase,  which  can  not  be  far  from  that  stated  above;  that  is,  roughly 
Or2Abt.  This  leads  to  the  assignment  of  the  composition  Ab, An r  to  the  ground- 
mass  plagioclase,  rendering  the  average  composition  AbtAn-,.  From  the  optical 
characters  of  the  augite  it  is  clear  that  none  of  the  acmite  molecule  is  present;  in  the 
readjustment  of  molecules  for  calculating  this  mineral  it  was  assumed  to  have  the 
composition  of  that  from  the  Ticchiena  albanose,  the  analysis  of  which  is  given 
later.  The  amount  of  AlaO3  thus  set  free  by  the  transfer  of  CaO  from  norma- 
tive anorthite  to  the  wollastonite  demanded  by  the  augite  is  just  about  sufficient  for 
the  small  amount  of  biotite  present. 

In  general,  the  modal  departures  from  the  norm  are  of  very  small  moment,  so 
that  the  mode  is  a  normative  one,  and  the  rock  may  be  described  as  normative 
salphyro-vulsinose. 

Occurrence. — The  best-known  locality  of  this  type  is  at  the  town  of  Bolsena  in 
the  Vulsinian  District,  and  it  probably  occurs  elsewhere  in  the  northern  part  of  this 
district.  Recently  rocks  of  this  type  have  been  described  by  De  Lorenzo  and  Riva 
from  the  Astroni  Volcano  in  the  Phlegrean  Fields,  and  the  same  type  is  found  in 
the  Auruncan  District,  as  in  the  valley  east  of  Casi  and  near  Torano,  both  near 
Teano. 

Name. — The  derivation  of  the  subrang  name  has  been  already  given,  and  that 
of  the  type  is,  of  course,  derived  from  the  locality  mentioned  above. 

In  the  prevailing  systems  of  classification  rocks  of  this  type  have  usually  been 
designated  trachyte,  though  they  differ  from  the  true  trachytes  chemically  in  their 
lower  silica  and  alkalis  and  higher  lime,  and  modally  in  the  presence  of  consider- 
able labradorite  or  anorthite;  and  this  abnormality  has  been  commented  on  by 
almost  every  petrographer  who  has  described  them.  On  these  grounds  I  proposed 
some  years  ago  that  they  be  separated  from  the  true  trachytes,  as  a  separate  group 
intermediate  between  the  trachytes  and  basalts,  and  that  they  be  called  vulsinite, 
this  name  having  been  applied  as  well  to  the  types  known  here  as  arsal  vulsinose 
and  arsal  and  bolsenal  ciminose.  While  the  separation  of  these  types  from  the  true 
trachytes  and  the  bestowal  of  a  distinctive  name  upon  them  has  been  criticized,  the 
close  accordance  between  the  more  numerous  analyses  which  we  now  have  of  them, 
and  their  very  marked  dissimilarity  in  chemical  composition  from  the  typical 
"trachytes"  of  the  older  systems,  as  revealed  by  their  norms  and  classification 
quantitatively,  would  seem  to  show  that  the  separation  and  the  use  of  the  new 
name  were  amply  justified. 


34  THE  ROMAN  COMAGMATIC  REGION. 

BOLSENAL  VULSINOSE.    I.  5.  2.  2. 

Megascopic  characters. — Light  gray,  sometimes  mottled  through  darker  schlieren,  compact 
porphyritic.  Feldspar  phenocrysts  rather  abundant,  i  to  5  mm.,  stout  prisms,  clear,  glassy. 
Augite  phenocrysts  few,  0.5  to  i  mm.,  prismatic,  black.  Biotite  phenocrysts  very  few,  0.5 
to  i  mm.,  thin  tabular,  bronzy  brown.  Groundmass:  light  gray,  aphanitic,  rough  fracture. 

Specific  gravity,  2.534  at  25°  C. 

Microscopic  characters. — Holocrystalline,  megaporphyritic,  dopatic,  mediophyric.  Pheno- 
crysts: about  20  per  cent,  orthoclase,  anorthite,  augite,  biotite.  Groundmass:  about  80  per 
cent,  trachytic  fabric,  orthoclase,  labradorite,  augite,  magnetite,  apatite,  titanite. 

Soda-orthodase,  Or9Ab4. — Phenocrysts:  about  10  per  cent,  i.o  to  5mm.,  subhedral, 
prismatic,  Carlsbad  twinning  common,  often  surrounded  by  a  mantle  of  later  orthoclase  sub- 
stance extending  irregularly  into  the  surrounding  groundmass.  Groundmass:  about  60  per 
cent,  0.05  to  0.2  mm.,  anhedral,  subprismatic  and  irregular,  arrangement  often  subparallel. 

Anorthite. — Phenocrysts:  about  6  per  cent,  0.5  to  3.0  mm.,  subhedral,  stout  prismatic, 
often  equant,  usually  multiply  twinned,  often  surrounded  by  later  mantle  of  orthoclase  like 
alkali-feldspar  phenocrysts.  Groundmass:  none,  except  as  below. 

Labradorite,  AbtAnj. — Phenocrysts  none.  Groundmass:  about  12  percent,  0.05  to  0.20  mm., 
subhedral,  prismatic  to  irregular,  arrangement  subparallel.  In  many  specimens  there  is  no 
difference  in  composition  between  the  plagioclase  of  the  phenocrysts  and  of  the  groundmass, 
both  having  about  the  composition  AbiAn2. 

Augite. — Phenocrysts:  about  2  per  cent,  0.2  to  i.o  mm.,  subhedral,  stout  prismatic  and 
fragmentary,  colorless  or  pale  greenish-yellow.  Groundmass:  about  5  percent,  o.oi  100.03  mm., 
anhedral,  prismatic  and  equant,  gray  or  pale  yellowish. 

Biotite. — Phenocrysts:  about  i  per  cent,  0.5  to  i.o  mm.,  subhedral,  thin  tabular,  brown, 
usually  altered.  Groundmass,  none. 

Magnetite. — Groundmass:  about  4  per  cent,  o.oi  to  0.2  mm.,  anhedral,  equant. 

Apatite  and  titanite. — Very  rare  accessories,  in  minute  crystals  of  usual  form. 

Chemical  composition  and  norm  as  on  p.  31. 

Type  specimen  from  below  castle,  Bolsena,  Lake  Bolsena,  Vulsinian  District. 

I.  5.  2.  2.    Viterbal  Vulsinose  [Leucite-Trachyte,  Viterbo  Type]. 

With  this  type  of  vulsinose  we  meet  the  first  of  the  long  and  interesting  series 
of  leucitic  rocks  for  which  the  Roman  Region  is  so  famous,  and  which  give  it  such  a 
strongly  marked  petrographic  character.  The  rocks  to  be  now  described,  further- 
more, exhibit  a  habit  which  is  one  of  the  best  marked  in  the  region  and  which  rivets 
the  attention  of  every  geologist.  The  assumption  of  this  leucitic  mode  by  a  per- 
salic  magma  is  of  some  interest,  as  leucite  rocks  are,  for  the  most  part,  of  the  dosalane 
or  salfemane  classes. 

Megascopic  characters. — The  rocks,  which  would  be  called  leucite-melaphyres 
in  the  field,  are  characterized  especially  by  the  abundance  and  size  of  the  phenocrysts 
of  leucite.  These  constitute  about  40  per  cent  of  the  rock  volume,  and  vary  in  diam- 
eter from  2  to  15  mm.,  with  an  average  of  about  8  mm.  These  leucites  are  highly 
euhedral  trapezohedra,  though  sometimes  fragmentary  through  breaking  up  during 
the  flow  of  lava.  They  are  lightly  spotted  with  black  inclusions  of  augite  and  glass, 
and  carry  as  well  some  inclusions  of  labradorite,  which  are  not  distinguishable  mega- 


PETROGRAPHY.  35 

scopically.  Owing  to  their  white  color,  their  size,  and  the  generally  rather  dark 
shade  of  the  groundmass,  these  phenocrysts  stand  out  very  prominently  and  give 
the  rocks  of  this  and  homologous  types  a  most  characteristic  appearance — so  much 
so  that  they  have  the  popular  name  of  "fish-eye"  (occhio  di  pesce).  Accompanying 
these  leucites,  but  very  inconspicuously,  are  a  few  much  smaller  phenocrysts  of  feld- 
spar (mostly  labradorite)  and  augite.  The  aphanitic  groundmass  is  a  rather  dark 
ash-gray. 

The  habit  of  these  and  homologous  types,  characterized  by  abundant,  large, 
euhedral  leucite  phenocrysts,  with  only  very  small  amounts  of  small  augite  pheno- 
crysts, and  a  rather  light  to  rather  dark  gray  aphanitic  groundmass,  may  be  called 
"viterboid."  Microscopically  the  groundmass  may  or  may  not  show  some  leucite, 
as  it  does  in  the  homologous  type  of  vicose,  the  relative  proportions  of  alkali-  and 
soda-lime  feldspars  in  the  groundmass  may  vary  considerably,  the  former  being 
usually  the  more  abundant,  and  the  fabric  may  be  more  or  less  trachytic.  But 
these  differences  are  microscopic,  and  it  would  not  seem  advisable  at  present  to 
regard  them  as  of  sufficient  importance  to  determine  different  habits. 

Microscopic  characters. — In  thin  section  the  large  leucites  show  the  usual 
double  refraction  and  twinning,  which  are  very  well  marked  unless  alteration  has 
set  in.  Inclusions  of  augite,  labradorite,  magnetite,  and  glass  are  not  uncommon. 
The  less  abundant  feldspar  phenocrysts  are  mostly  of  orthoclase,  with  some  of  lab- 
radorite. They  are  in  the  form  of  stout  subhedral  prismoids,  often  with  numerous 
glass  inclusions,  mostly  clustered  toward  the  center.  The  augite  phenocrysts  are 
mostly  well  formed  crystallographically,  and  are  of  the  usual  very  pale  gray,  with 
only  a  slight  tinge  of  green  and  non-pleochroic.  The  groundmass  is  holocrystalline, 
composed  in  very  large  part  of  small  orthoclase  prismoids,  whose  arrangement  is 
diverse,  forming  a  felted  fabric.  With  these  are  small  quantities  of  minute  augite 
prismoids,  some  biotite  flakes  interstitial  between  the  feldspar  laths,  magnetite 
grains,  and  an  occasional  small  apatite  and  titanite.  A  few  of  the  specimens  which 
are  of  this  type  contain  small  amounts  (not  more  than  2  per  cent)  of  leucite  in 
the  groundmass,  as  very  small,  clear,  equant  euhedra,  in  this  respect  being  transi- 
tional toward  the  viterbal  vicose  to  be  described  later. 

Chemical  composition. — Two  analyses  were  made  of  viterbal  vulsinose  and 
are  presented  below,  along  with  one  of  bolsenal  vulsinose  for  comparison.  The 
second  of  these  was  made  and  published  some  years  ago,  though  additional  deter- 
minations of  minor  constituents  have  been  made  since.  It  differs  from  the  other 
in  several  respects,  notably  in  lower  alkalies  and  alumina,  and  higher  oxides  of  iron, 
lime,  and  magnesia.  As  will  be  seen  when  the  mode  is  under  discussion,  there  is 
strong  reason  for  the  belief  that  the  sample  analyzed  was  not  representative  of  the 
rock,  but  contained  less  of  the  leucite  phenocrysts  and  more  of  the  groundmass. 
For  this  reason  the  analysis  of  the  Grignano  rock  is  to  be  regarded  as  the  most 
typical. 


THE  ROMAN  COMAGMATIC  REGION. 

Chemical  Composition  of  Viterbal  Vulsinose  [Leucite-trachyte]. 


I. 

II. 

III. 

IV. 

SiOa      

c6.  TO 

o.  o?6 

CC    T7       O 

eg  og 

rg     ..g 

A12O3  

20.  7? 

.  20? 

19.60 

IO2 

19    II 

i8.<;6 

Fe,O,  . 

I.  71 

.on 

1.  27 

O2O 

7.  ce 

2    O? 

FeO  

2.  IQ 

.031 

2.  74 

0*8 

I  .  OO 

-,  r6 

MeO  . 

I-  It 

.020 

i.<8 

040 

I    OS 

O    ?7 

CaO      

1.  C7 

.  063 

2.  72 

067 

•2.  76 

OO 
2     6O 

Na2O       

2.86 

.046 

2    27 

O77 

2    84 

KjO  

10.47 

.  112 

o.  c8 

IO2 

8.86 

IO    47 

HaO+  

^ 
o.  70  / 

O.  C4  ) 

H2O—   

f 
o.  30  S 

0.99 

J*  [ 
O.  II   ) 

0.24 

CO2  

none 

none 

none 

TiO2  

0.6"; 

.008 

o.  69 

009 

0.82 

o  70 

P,Or. 

o.  24 

.OO2 

o.  20 

OOI 

o.  20 

o  20 

MnO     

n.  d. 

n.  d. 

n  d 

100.73 

99.82 

99.92 

IOO.2I 

I.  Viterbal  vulsinose  [leucite-trachyte].     Sorgente  di   Grignano,  southwest  of  Lake  Vico, 

Ciminian  District.     H.  S.  Washington,  analyst. 

II.  Viterbal  vulsinose  [leucite-trachyte].  Below  San  Rocco,  Monte  Vico,  Ciminian  Dis- 
trict. H.  S.  Washington,  analyst.  Jour.  Geol.,  V,  1897,  p.  370.  Additional 
determinations  here. 

III.  Bolsenal  vulsinose  [vulsinite].     Bolsena,  Vulsinian  District.     (Cf.  p.  31.) 

IV.  Bagnoreal  vulsinose  [leucite-trachyte].     Monte  San  Antonio,  Auruncan  District,      vom 

Rath,  analyst.      Zeits.  d.  'd.  geol.  Ges.,  XXV,  1873,  p.  244.    Fe2O3,  FeO,  TiO2, 
and  PaOs  estimated  by  H.  S.  W. 


Norms. 


Or  

Ab 

c.76  }• 

An 

.     12  .  CI    1 

Ne     

Q.04 

Di  

2  .  44.  / 

Ol  

2       2   \ 

Mt    

2  55  I 

11 

1  .  22   \ 

Ap  .  . 

0.68 

Rest  

99-79 

I  .  OO 

100.79 


Class. 


I. 
80.54 

9-94 
4.86 

3-17 
0.68 


Ratios. 


Sal 


II. 


90.48 


56-71) 

"•53  [ 

82.97 

I4-73) 

• 

4.26 

4.26, 

2.22  ) 

2-95  ) 

5-i7 

4.64? 
i-37) 

6.91 

0.40 

0.40 

98.81 

0.99 

87.23 


11.58 


Order 


'Fern 
F 
'L 


99.80 

I. 

-9.72 


II. 

7-54 


Rang.  .  . 
Subrang 


CaO' 
K2O' 


'Na2O' 


=8.10         19-74 
=3.51  2.62 

•2.43          2.76 


PETROGRAPHY. 


37 


Comparison  of  the  analysis  of  this  leucitic  type  (I)  with  that  of  the  non-leucitic 
(III)  is  of  interest.  While  they  are  closely  similar  in  the  figures  for  alumina,  iron 
oxides,  magnesia,  lime,  and  soda,  the  silica  of  I  is  lower  by  about  2  per  cent,  and 
potash  higher  by  about  1.6  per  cent.  These  differences  are  not  very  great,  but  they 
are  concordant,  and  sufficient  to  have  determined  (it  may  be  concomitantly  with 
the  physical  conditions)  the  formation  of  modal  leucite  in  the  one  and  its  absence  in 
the  other.  At  the  same  time,  it  must  be  understood  that  modal  leucite  could  have 
formed  from  the  magma  of  III,  as  is  clear  from  consideration  of  the  norm,  though 
only  to  a  comparatively  small  extent,  limited  by  the  small  amounts  of  normative 
nephelite  and  olivine  which  this  type  shows. 

The  analysis  in  IV  has  been  corrected  by  me  from  vom  Rath's  original  one  in 
regard  to  the  oxides  of  iron,  titanium,  and  phosphorus,  the  corrections  being  based 
on  an  analysis  of  mine  of  a  leucite  rock  from  the  same  locality.  It  will  be  discussed 
later. 

Mode. — The  measured  mode  of  I  was  determined  both  megascopically  and 
microscopically,  the  phenocrysts  of  leucite  having  been  measured  on  the  surfaces 
of  a  good- sized  hand  specimen  and  those  of  labradorite  and  augite,  with  the  con- 
stituents of  the  groundmass,  in  the  thin  section.  For  the  calculated  mode  the  com- 
position of  augite  was  assumed  to  be  that  occurring  in  the  Ticchiena  albanose,  as  in 
the  preceding  case,  and  that  of  the  biotite  to  be  about  equal  parts  of  leucite  and 
olivine.  This  last  is  not  strictly  correct,  but  the  error  will  be  slight  in  view  of  the 
small  amount  of  this  mineral  present. 


CALCULATED. 

MEASURED. 

Soda-orthoclase,  OrgAb7  

39-6 
17-5 
31-4 
5-3 

2-3 

3-2 
0.7 

Vol.  %.     Sp.  gr. 
42.0    X   2.6    = 
ii.  o   X  2.7   = 
38.0   X   2.5    = 
5-4   X  3.3    = 

2.0    X    2-9     = 

1.6  X  5-2   = 

Wt.  %. 
109.20     41.1 

29  .  7O       I  I  .  2 

95-00    35.7 
17.82      6.7 

5  .  80         2.2 

8.32      3-1 

Labradorite,  AbiAna  

Leucite   

Augite  

Biotite  

Magnetite  

Apatite    

100.  0 

IOO.O 

265.84  100.0 

It  will  be  seen  that  the  correspondence  is  fairly  close  for  most  of  the  constitu- 
ents, especially  when  the  very  fine  grain  of  the  groundmass  is  taken  into  considera- 
tion. The  only  marked  discrepancies  are  in  the  amounts  of  labradorite  and  leucite, 
which  are  respectively  lower  and  higher  in  the  measured  mode.  The  former  is  prob- 
ably due  in  part  to  the  presence  of  some  labradorite  inclusions  in  the  leucite  pheno- 
crysts, which  were  not  reckoned  in,  and  also  in  part  to  the  possible  presence  of  some 
lime-soda  feldspar  among  the  groundmass  laths.  The  measured  leucite  must,  of 
course,  be  diminished  by  the  amount  of  labradorite  which  it  includes,  as  well  as  by 
the  small  quantities  of  augite  and  glass  inclusions,  whose  exact  amount  could  not 


38  THE  ROMAN  COMAGMATIC  REGION. 

be  definitely  ascertained,  as  most  of  the  leucites  were  much  broken  in  the  sections 
and  their  inclusions  lost.  Of  the  two  modes,  that  calculated  from  the  norm  is 
regarded  as  the  more  reliable  and  the  nearer  approximation  to  the  true  composition. 

From  these  it  is  clear  that  the  modal  deviations  from  the  norm  due  to  alferric 
minerals  are  inconsiderable,  but  that  the  essential  difference  lies  in  the  modal 
leucite  replacing  a  negligible  amount  of  normative  nephelite.  The  relations  of  mode 
and  norm  are  of  especial  interest  as  illustrative  of  the  possibility  that  a  rock 
belonging  to  a  perfelic  order  may  carry  modally  about  30  per  cent  of  the  lenad 
leucite.  This  will  be  adverted  to  in  a  later  discussion.  From  the  description  of 
the  rock  and  the  relation  of  the  mode  to  the  norm,  the  type  may  be  described  as  a 
leucite-salphyro-vulsinose. 

The  mode  of  II  was  less  satisfactorily  estimated  by  Rosiwal's  method,  and  as 
thus  determined  agrees  far  less  closely  with  that  calculated  from  the  norm.  The 
leucite  phenocrysts  were  determined  by  measurements  on  the  surfaces  of  the  hand 
specimen,  and  those  of  feldspar  and  augite  phenocrysts  in  thin  section,  both  as 
compared  with  the  groundmass  as  a  whole.  Attempts  to  estimate  the  relative 
amounts  of  the  minerals  in  this  last  were  unsatisfactory,  because  of  their  very  smal] 
size  and  the  consequent  large  error  due  to  overlapping.  As  thus  determined  the 
mode  may  be  stated  as  follows: 

Vol.  %       Sp.  gr.  Wt.  % 

Orthoclase  phenocrysts 1.1X2.6="  2.9  i.i 

Labradorite  phenocrysts ....   14.1   X  2.7   =  38.1  14. 7 

Leucite  phenocrysts 38.1   X  2.5   =  95-3  36-8 

Augite  phenocrysts 1.4   X  3.3   =  4.6  1.8 

Groundmass 45-3   X  2.6   =  117.8  45-6 

100.0  258.7       100.0 

Comparing  this  with  the  Rosiwal  mode  of  the  Grignano  rock,  it  is  seen  that  the 
amounts  of  leucite  phenocrysts  in  both  are  almost  alike,  and  as  these  could  be 
measured  with  considerable  accuracy,  it  must  be  assumed  that  the  measured  amounts 
of  this  mineral  are  very  close  to  the  truth,  though  slight  corrections  must  be  made 
for  the  small  quantities  of  inclusions  which  the  large  phenocrysts  carry,  and  which 
it  was  not  practicable  to  estimate. 

The  mode  was  also  calculated  from  the  norm  by  the  usual  process  of  successive 
readjustments,  all  the  normative  olivine  being  changed  to  hypersthene,  which  enters 
into  modal  augite,  and  the  normative  nephelite  taking  silica  enough  to  form  albite, 
which  enters  into  modal  alkali- feldspar  and  labradorite.  In  this  way  we  obtain 
the  maximum  amount  of  molecules  of  minerals  as  highly  silicated  as  possible,  leav- 
ing as  little  silica  as  possible  to  combine  with  potash  in  orthoclase  and  leucite,  the 
amounts  of  which  are  thus  theoretically  fixed  and  can  be  calculated  by  the  equa- 
tions given  in  the  "Quantitative  Classification"  on  page  194.  We  must  thus 
obtain  the  maximum  amount  of  leucite  capable  of  formation  from  the  magma, 
as,  if  more  were  assumed  to  be  present,  some  silica  would  necessarily  be  set  free, 
which  is  contrary  to  the  actual  facts. 


PETROGRAPHY.  39 

In  this  way  the  following  calculated  mode  was  obtained: 

Orthoclase,  Or, Ab2 47 . 85 

Labradorite,  Abx  Ana 19 . 91 

Leucite 17.22 

Augite 8.21 

Ores 5  •  24 

Apatite o. 40 

98.83 
H2O,  etc 0.99 

99.82 

It  is  clear  that  the  discrepancy  between  the  observed  and  the  calculated  amounts 
of  leucite,  36.8  and  17.2  per  cent,  can  not  be  explained  by  the  corrections  needed 
for  inclusions,  and  it  has  been  shown  above  that  the  larger  amount  of  leucite  as 
revealed  by  Rosiwal's  method  must  be  close  to  the  truth.  As,  furthermore,  by  no 
possible  means  can  more  leucite  be  calculated  from  the  magma  without  setting 
silica  free,  and  there  is  no  reason  to  believe  that  the  analysis  is  seriously  incorrect, 
it  follows  that  the  only  allowable  explanation  of  the  discrepancy  is  that  the  sample 
analyzed  did  not  correspond  to  the  rock  as  a  whole,  but  contained  an  unduly  large 
amount  of  groundmass  and  an  unduly  small  amount  of  the  leucite  phenocrysts.* 
That  this  explanation  is  the  correct  one  is  the  more  probable  when  it  is  con- 
sidered that  the  analysis  was  made  by  me  some  eight  years  ago,  when  the 
importance  of  crushing  up  a  sufficiently  large  piece  of  rock  had  not  impressed  itself 
upon  me,  and  when  the  coarsely  porphyritic  character  of  the  rock  and  of  the  brittle- 
ness  of  the  leucite  phenocrysts  are  taken  into  account.  If  a  rather  small  piece  was 
taken,  it  would  be  probable  that  the  sample  analyzed  would  contain  relatively  more 
groundmass  than  the  rock  itself.  This  conclusion  is  borne  out  by  the  fact  that  the 
modes  of  the  San  Rocco  specimen  show  considerably  more  of  the  groundmass  con- 
stituents than  in  the  other  specimen.  Attention  is  called  to  this  otherwise  rather 
unimportant  point,  as  it  serves  to  emphasize  a  feature  of  the  analysis  of  rocks — the 
proper  preparation  of  the  sample — on  which  I  have  laid  stress  elsewhere.f  In  view 
of  the  conclusions  above,  the  analysis  (II)  of  the  San  Rocco  vulsinose  can  not  be 
considered  as  representative  of  the  rock  magma,  and  must  be  disregarded  in  future 
discussions. 

Occurrence. — This  type  has  been  found  with  certainty  only  in  the  Ciminian 
District,  where  it  is  very  abundant,  many  of  the  lava  flows  of  the  Vico  Volcano 
belonging  here.  Among  localities  where  especially  good  specimens  were  collected 
may  be  mentioned  several  flows  in  the  east  inner  wall  of  Monte  Vico  below  San 
Rocco,  flows  near  San  Martino,  northwest  of  Lake  Vico,  especially  above  the 
church  and  below  the  town  at  the  Contrada  di  Merlano,  and  southwest  of  Lake 
Vico,  as  at  the  Sorgente  di  Grignano  and  the  Ponte  delle  Cese. 

*  As  the  silica  percentage  of  leucite  is  55.05,  and  therefore  close  to  that  of  the  rock,  the  removal  of  leucite  would 
not  materially  affect  the  silica  shown  by  the  analysis. 

tH.  S.  Washington,  P.  P.  14,  U.  S.  G.  S.,  p.  18,  1903;  and  The  Chemical  Analysis  of  Rocks,  New  York, 
1904,  p .  47- 


40  THE  ROMAN  GOMAGMATIC  REGION. 

The  type  may  also  occur  in  the  Auruncan  District.  The  analysis  of  a  "leu- 
cite-trachyte "  by  vom  Rath  given  in  column  iv  is  that  of  a  vulsinose  and  with  a 
leucitic  mode.  A  rock  with  prominent  leucite  phenocrysts  collected  by  me  at  this 
locality  shows  much  less  silica  (51.20)  and  falls  in  vicose  (II.  6.  2.  2),  and  is  clearly 
not  the  same  as  vom  Rath's  rock,  as  his  silica  determination  can  not  be  so  greatly  in 
error.  His  description  is  not  very  clear  or  detailed,  but  it  may  be  gathered  that  the 
mode  is  probably  closely  like  that  of  the  bagnoreal  ciminose  and  vicose,  to  be 
described  later,  in  which  the  leucite  phenocrysts  are  much  less  abundant  and  less 
prominent,  and  with  more  numerous  augite  phenocrysts.  I  have  therefore  assigned 
vom  Rath's  rock  provisionally  to  the  type  of  bagnoreal  vulsinose,  though  this  needs 
further  investigation.  The  descriptions  of  Bucca  lead  one  to  believe  that  true 
viterbal  vulsinose  is  quite  abundant  in  the  Auruncan  District,  as  at  the  localities 
of  Valogno  Piccolo  and  below  Orchi.  I  did  not  visit  the  former  locality,  and  the 
rocks  which  I  collected  near  the  latter  village  are  not  of  this  type. 

Name. — The  type  adjective,  viterbal,  is  derived  from  the  city  of  Viterbo,  the 
chief  town  of  the  Ciminian  District.  In  prevailing  classifications  rocks  of  this 
type  have  gone  under  very  different  names.  If  we  disregard  the  small  amount  of 
labradorite,  which  is  always  subordinate  to  the  orthoclase,  the  rocks  would  be  called 
by  Zirkel  leucite-trachytes,  and  by  Rosenbusch  leucite-phonolites.  Of  these  two, 
that  of  Zirkel  is  the  better,  in  my  opinion,  for  reasons  given  elsewhere.  Some 
petrographers,  however,  regard  the  presence  of  the  labradorite  as  of  more  impor- 
tance than  that  of  the  more  abundant  orthoclase,  and  therefore  call  these  rocks  leucite- 
tephrites.  Of  this  appellation  it  may  be  remarked  that  it  is  at  variance  with  the 
quantitative  relations  of  the  two  feldspars,  ignoring  that  which  is  most  abundant 
and  laying  special  stress  on  the  subordinate  one.  It  may  also  be  noted  that  these 
rocks  differ  widely  from  others  of  the  region  to  which  the  name  of  leucite-tephrite 
is  applied  with  as  much  logical  reason  as  the  prevailing  systems  are  capable  of. 
Under  the  prevailing  systems,  however,  there  is  no  means  of  distinguishing  this 
well-marked  type  from  others  in  which  the  leucite  is  not  decidedly  porphyritic. 
The  characters  are  so  well  marked  and  so  easily  recognizable,  and  these  rocks  are 
so  important  in  the  petrography  of  the  region,  that  they  would  seem  to  be  deserving 
of  a  separate  name.  For  this  the  term  viterbite  may  be  used. 

I.  5.2.  2.  Pallanzanal  Vulsinose  [Leucite-Trachyte,  Pallanzana  Type]. 

This  type  is  intermediate  between  the  bolsenal  and  viterbal  types.  It  resembles 
the  former  in  the  rather  small  and  rare  feldspar  phenocrysts  and  the  latter  in  the 
presence  of  phenocrysts  of  leucite,  though  these  last  are  much  smaller  and  far  less 
abundant  than  in  the  rocks  just  described. 

The  rocks  are  all  more  or  less  decomposed,  being  so  friable  that  it  was  impos- 
sible to  obtain  good  specimens,  and  it  is  noteworthy  that  all  writers  mention  the 
uniformly  altered  condition.  On  this  account  no  analysis  was  made  of  them,  so 
that  their  exact  magmatic  position  is  not  sure.  Microscopic  study,  however,  ren- 


PETROGRAPHY.  41 

dered  it  evident  that  at  least  the  majority  of  the  occurrences  fall  in  vulsinose,  so 
that,  as  a  description  is  called  for  on  account  of  their  importance  in  the  Ciminian 
District,  it  may  be  inserted  here. 

The  rocks  are  light  gray,  megaphyric,  and  dopatic  to  perpatic.  The  majority 
of  the  phenocrysts  are  of  feldspar,  mostly  of  orthoclase,  but  with  about  half  as  much 
labradorite.  These  are  up  to  20  or  30  mm.  in  length  and  euhedral  to  subhedral 
tabular.  The  feldspar  phenocrysts  constitute  from  10  to  15  per  cent  of  the  rock. 
Phenocrysts  of  leucite  are  less  numerous,  making  up  not  more  than  about  5  per  cent 
of  the  rock.  They  vary  from  5  to  10  mm.  in  diameter,  are  anhedral  equant,  and 
always  a  dull  white  through  kaolinization.  Very  few  phenocrysts  of  augite  and 
biotite  are  also  present,  of  about  the  size  of  the  leucites. 

The  groundmass  is  either  very  fine  grained  or  aphanitic,  and  is  quite  often 
somewhat  vesicular,  though  never  scoriaceous.  It  is  also  holocrystalline  and  closely 
resembles  that  of  the  bolsenal  type,  composed  in  very  large  part  of  small  laths  of 
orthoclase,  which  have  a  subparallel  arrangement.  With  these  are  a  very  few 
small  (0.05  to  o.oi  mm.),  round  spots  of  colorless,  isotropic  substance,  which  is  prob- 
ably to  be  referred  to  leucite.  Their  amount,  however,  is  negligible.  There  are 
the  usual  small  prismoids  and  irregular  grains  of  colorless  augite,  and  fewer  of 
magnetite,  but  none  of  the  interstitial  biotite  seen  in  the  preceding  phase.  Here 
this  mineral  seems  to  have  developed  entirely  as  phenocrysts. 

The  occurrence  of  these  rocks  is  confined  to  the  Ciminian  District,  where  they 
are  rather  common,  being  known  locally  as  "petrisco."  The  typical  "petrisco"  is 
best  developed  in  the  northeastern  part  of  the  district,  where  it  forms  extensive 
flows  from  the  Vico  Volcano.  The  largest  of  these  originated  apparently  on  the 
north  flank  of  Vico,  and  passed  northward  around  Monte  Pallanzana,  which 
divided  it  into  two  branches.  From  this  flow  characteristic  specimens  were  obtained 
by  me  at  the  Villa  Lante  near  Bagnaia  and  at  Cavorcie  on  the  north  branch  and 
at  I  Cappuccini  on  the  south  branch,  as  well  as  from  other  localities.  Mercalli 
describes  several  flows  of  "petrisco"  on  the  east  and  south  of  Monte  Vico,  between 
Capranica  and  Ronciglione,  but  whether  they  are  strictly  of  this  type  or  not  is 
uncertain,  as  I  was  unable  to  visit  them. 

As  stated  above,  no  analysis  was  made  of  this  type,  on  account  of  the  weathered 
condition  of  all  the  specimens,  so  a  formal  description  is  uncalled  for. 

I.  6-5.  2.  3.    Paglial  Procenose-Pulaskose  [Leucite-Trachyte,  PagliaTypej. 

Megascopic  characters. — In  the  hand  specimen  the  rocks  of  this  type  are  much 
like  those  of  the  type  of  sabatinal  beemerose  to  be  described  later,  the  main  differ- 
ence being  the  somewhat  darker  groundmass.  They  are  medium  gray  in  color, 
very  compact,  and  markedly  porphyritic.  Leucite  phenocrysts  are  not  very  numer- 
ous, making  up  but  about  one-tenth  of  the  rock,  but  they  are  conspicuous  and  quite 
large,  from  5  to  20  mm.  in  diameter.  Most  of  them  show  crystal  planes,  but 
all  are  more  or  less  fragmentary,  so  that  completely  bounded  trapezohedra  are  very 


42  THE  ROMAN  COMAGMATIC  REGION. 

rare.  Their  color  is  inclined  to  yellowish,  the  luster  is  apt  to  be  somewhat  waxy, 
and  they  are  mostly  quite  free  from  inclusions.  There  are  also  very  few  large  (10  to 
20  mm.  long),  stout  prismoids  of  feldspar,  which  the  microscope  proves  to  be  of 
orthoclase,  and  some  smaller  ones  of  what  the  thin  section  shows  to  be  labradorite. 
In  addition  are  very  few  and  small  prismoids  of  black  augite,  so  inconspicuous  as 
almost  to  escape  notice.  The  groundmass  is  a  rather  dark  ash  gray,  and  quite 
aphanitic. 

Microscopic  characters. — In  thin  sections  the  larger  leucite  phenocrysts  have 
generally  fallen  out,  but  the  fragments  left  show  the  usual  double  refraction  and 
twinned  structure.  One  or  two  sections  of  the  large  feldspar  phenocrysts  show  that 
they  are  of  orthoclase,  undoubtedly  sodic,  while  the  smaller  ones  are  of  labradorite, 
about  Abx  An2.  These  last  are  much  twinned,  and  are  usually  gathered  into  clusters, 
whose  numbers  are  greater  than  those  of  the  orthoclase  phenocrysts.  The  pheno- 
crysts of  augite  are  subhedral  or  anhedral,  mostly  in  stout  prismoids  or  fragments, 
and  of  the  usual  pale- gray  tint. 

The  groundmass  is  holocrystalline,  and  the  fabric  is,  on  the  whole,  trachy- 
tic,  though  not  as  typically  so  as  in  many  other  types,  on  account  of  the  presence  of 
considerable  numbers  of  formless  anhedra  of  orthoclase  and  rounded  leucites.  It 
is  composed  very  largely  of  feldspar,  for  the  most  part  in  slender  prisms  or  laths, 
of  which  the  majority  are  of  orthoclase  with  fewer  of  labradorite,  though  it  was 
impossible  to  arrive  at  any  satisfactorily  accurate  estimate  of  the  relative  amounts. 
As  noted  above,  anhedral  individuals  of  orthoclase  also  occur.  Leucite  is  present 
in  considerable  amount  as  small,  round  anhedra,  defined  by  the  rings  of  augite 
and  magnetite  grains  and  feldspar  laths  surrounding  them,  and  identifiable  by  the 
faint  double  refraction.  Scattered  through  the  mass  are  the  usual  small  prismatic 
anhedra  of  colorless  augite,  equant  anhedra  of  magnetite,  and  rare  apatite  prisms. 
Neither  nephelite  nor  glass  could  be  detected,  and  the  sodalite  minerals  seem  to  be 
wholly  absent. 

Chemical  composition. — For  the  analysis  a  fair-sized  hand  specimen  was  crushed 
and  sampled,  in  order  to  obtain  a  representative  powder,  on  account  of  the  large 
size  of  the  leucite  phenocrysts.  With  the  analysis  of  mine  is  given  one  by  Ric- 
ciardi  of  a  rock  also  from  Proceno,  called  a  leucite-tephrite  by  Klein,  whose  descrip- 
tion agrees  in  the  main  with  that  of  the  specimen  analyzed  by  me.  There  is  also 
given  an  analysis  by  Riva  of  a  bolsenal  vulsinose-pulaskose  (vulsinite)  from  the 
Astroni  Volcano  in  the  Phlegrean  Fields.  • 

The  analysis  itself  does  not  call  for  special  remark,  except  that  the  consider- 
able amount  of  BaO  and  the  presence  of  small  quantities  of  zirconia,  strontia,  and 
the  "rare  earths"  may  be  noted.  These  last  were  determined  with  care  by  Hille- 
brand's  method,*  and  there  is  probably  little  doubt  that  they  exist  in  similarly  small 
amounts  in  other  rocks  of  the  region,  as  they  were  also  found  to  be  present  in  a  more 
femic  rock. 

*  W.  F.  Hillebrand,  Bull.  176,  U.  S.  Geol.  Surv.,  p.  77,  1900. 


PETROGRAPHY. 

Chemical  Composition  of  Paglial  Procenose-pulaskose  [Leucite-lrachyte]. 


43 


I. 

II. 

III. 

<MO, 

SS.O7     o 

Ql8 

C7.6o    o 

060 

A12O3                 

20.  83 

204 

10.43 

IOO 

Fe,O, 

2.  12 

OI2 

2.40 

016 

FeO    

I.  QQ 

028 

1.  02 

026 

1  -y.j 

8    17 

MeO     . 

I.OO 

02  <; 

1.  06 

027 

CaO        

•3.  17 

060 

4.17 

07? 

4  80 

NaaO       

4.OO 

06? 

3.  CC 

OS  7 

K2O               

8.6<: 

OQ? 

8.71 

OQ3 

H,O+  

O.77 

0.32 

HjO—  

O.  SO 

O.  ?2 

1-54 

CO,    

none 

TiOj          

0.82 

OIO 

0.46 

OO6 

ZrOa   

0.04 

p,O, 

O.  K) 

OOI 

O.  2O 

OOI 

trace 

SO3                         .... 

trace 

o  64 

Cl    

O.O4 

trace 

(Ce  Di),O, 

o.  03 

MnO  

n.d. 

O.23 

OO3, 

0.44 

BaO  

O.2O 

SrO  

O.O6 

99-73 

100.50 

100.27 

I.  Paglial   procenose-pulaskose  [leucite-trachyte].     Below  castle,   Proceno,  N.  W.  of  Lake 

Bolsena,  Vulsinian  District.     Washington,  analyst. 
II.  Bolsenal  vulsinose-pulaskose  [vulsinite].     Rotondella,  Astroni  Volcano,  Phlegrean  Fields. 

Riva,  analyst.     De  Lorenzo  and  Riva,  Alt.  Ace.  Sc.  Nap.,  XI,  1902,  p.  52. 
III.  Pagb'al  pulaskose?  ["leucite-tephrite"].     Below  castle,  Proceno.    Ricciardi,  analyst.    Klein, 
Neu.  Jahrb.,  B.  B.  VI,  1889,  p.  26. 


Norm  of  I. 

Or 51.71) 

Ab 14- 15  [78- 65) 

An 12.79)  ^89.44 

Ne i°-79     io-79) 


Norm  of  II. 

5i-7i 

I-  8i.95) 

87.77 


Di. 
01.. 
Mt. 
II... 

Ap.. 


2.22 
1-35 

1.52 
O.  34 


3-57 
4-54 


11.12 
5-82 
6.83 


5.82 
6.83 


3"7I[    4-62 
0.91  )    * 

0.36      0.36 


11.79 


Rest. 


97.89 
1.69 

99-58 


99-58 
1-09 

100.67 


Class  . 
Order. . 


Subrang. 


Ratios. 

Sal 
Fern 
F 
L 

K2O'+Na20' 
CaO' 

K2O' 
Na,O' 


I. 
10.56 

7.29 
3-43 
1-43 


II. 

7.44 

14.08 


1.63 


44  THE  ROMAN  COMAGMATIC  REGION. 

From  the  figures  of  the  norm  and  the  ratios  it  is  seen  that  the  rock  falls  well 
within  the  persalane  class  and  the  domalkalic  rang,  and  fairly  well  within  the  sodi- 
potassic  subrang,  though  somewhat  close  to  the  dopotassic  border.  But  it  is  almost 
exactly  on  the  line  between  the  perfelic  and  lendofelic  orders,  falling  in  order  5  or  6 
according  to  whether  the  small  amount  of  apatite  is  calculated  in  the  norm  or  not. 
It  is  therefore  eminently  a  transitional  one  belonging  strictly  in  pulaskose,  I.  5.  2.  3, 
but  very  close  to  the  border  of  the  subrang  I.  6.  2.  3,  as  yet  unnamed,  which  may 
be  called  procenose,  from  the  locality  of  the  specimen  analyzed.  The  magmatic 
name  then  will  be  procenose-pulaskose. 

The  analysis  of  the  Astroni  rock  in  II  resembles  the  other  very  closely,  except 
in  containing  2.5  per  cent  more  silica.  This  larger  amount,  by  increasing  the  quan- 
tity of  normative  albite  and  diminishing  that  of  normative  nephelite,  removes  the 
type  well  away  from  the  border  of  order  6,  while,  like  the  preceding,  it  falls  well 
within  class  I  and  rang  2.  On  the  other  hand,  the  ratio  of  soda  to  potash  brings  it 
very  near  the  line  of  dopotassic  subrang  2,  so  that  this  type  must  be  regarded  as  a 
vulsinose-pulaskose.  It  will  be  remembered  that  the  other  rocks  of  the  Astroni 
Volcano  fall  in  vulsinose,  one,  indeed,  being  strictly  a  pulaskose-vulsinose. 

The  analysis  by  Ricciardi  in  III  must  be  of  a  rock  almost,  if  not  quite,  the 
same  as  that  analyzed  by  me,  as  Klein's  description  agrees  with  mine  in  all  essen- 
tial respects,  and  the  town  of  Proceno  is  built  upon  but  one  thick  lava  flow,  from 
which  the  two  specimens  must  have  come.  It  will  be  seen  that  here,  as  in  so  many 
other  of  the  same  chemist's  analyses,  the  amounts  of  the  alkalis  reported  are  quite 
irreconcilable  with  the  mode  of  the  rock.  Thus  the  potash  reported  would 
allow  the  formation  of  but  18.35  °f  orthoclase  or  14.39  of  leucite,  and  the  silica  is  so 
high  that  it  can  not  be  satisfied  by  the  bases  present,  and  the  rock  would  necessarily 
contain  considerable  amounts  of  quartz.  Indeed,  the  discrepancy  is  so  marked 
that  Klein  comments  on  the  apparently  low  figure  for  alkalis. 

Mode. — Estimation  of  the  mode  of  the  paglial  procenose-pulaskose  could  be 
carried  out  by  Rosiwal's  method  only  as  far  as  the  phenocrysts  and  the  dark  minerals 
were  concerned.  The  feldspar  laths  of  the  groundmass  were  far  too  fine  and  con- 
fused in  arrangement  to  allow  of  separate  estimation  of  the  orthoclase  and  the  labra- 
dorite.  The  amount  of  the  large  leucite  phenocrysts  (10.4  per  cent  by  volume)  was 
determined  in  the  hand  specimen,  while  the  thin  section  was  used  for  those  of  the 
groundmass  and  the  other  constituents.  The  results  may  be  thus  tabulated: 


Orthoclase  phenocrysts.  . 
Labradorite  phenocrysts. 
Leucite  

Vol.  %.      Sp.  gr. 
1.1X2.6=       2.9 
2.9   X  2.7   =       7.8 
16.6   X  2.5    =     4-i-S 

Wt.%. 
i.i 

2-9 

i=;.6 

Augite  

4.0    X     ?.  3     —       l6    2 

6  i 

Magnetite  

1  .4    X    "\  .  2     —          73 

2    8 

Feldspar,  groundmass.  .  . 

.     73.1   X  2.6  =  190.0 

100.0  265.7     100.0 

In  calculating  the  mode  from  the  norm  it  will  be  found  that  if  all  the  nephelite 
is  assumed  to  take  up  silica  to  form  modal  albite  there  must  result  36.2  per  cent  o 


PETROGRAPHY.  45 

leucite  and  only  5.6  per  cent  of  orthoclase.  These  amounts  are  obviously  incorrect, 
as  the  amount  of  leucite  shown  by  Rosiwal's  method  can  not  be  so  far  from  the  truth, 
since  this  mineral  is  mostly  in  the  form  of  phenocrysts  and  is  easily  measurable.  If 
we  then  assume  that  the  amount  of  leucite  is  that  given  above  16.6  per  cent  and  that 
all  the  olivine  molecules  enter  as  hypersthene  into  the  augite,  we  shall  obtain  the 
following  results: 

Orthoclase,  Or5Ab3 48 . 3 

Labradorite,  Abj  An2 18.3 

Leucite 16.8 

Nephelite 6.5 

Augite 6.2 

Ores 3.9 

IOO.O 

This  calculated  mode  may  be  regarded  as  closely  correct,  though  it  differs  from 
the  microscopic  description  in  the  presence  of  nephelite.  The  amount  of  this, 
however,  is  small,  and  it  could  easily  be  overlooked  among  the  closely  packed  feld- 
spar laths.  After  attention  was  directed  to  its  probable  presence,  examination  under 
high  powers  revealed  some  colorless  interstitial  cement,  with  weak  birefringence, 
which  may  well  be  this  mineral. 

The  deviation  of  the  mode  from  the  norm  is  therefore  considerable,  but  is 
especially  marked  in  the  presence  of  abundant  leucite,  replacing  some  of  the  norma- 
tive orthoclase  and  nephelite,  much  of  the  latter  becoming  albite  and  entering  the 
feldspars.  The  readjustments  due  to  alferric  minerals  are  negligible.  The  rock 
may  therefore  be  described  as  a  leucite  salphyro-procenose-pulaskose. 

Occurrence. — The  most  important  occurrence  of  the  type  of  paglial  procenose- 
pulaskose  is  the  very  extensive  flow  which,  issuing  from  the  Latera  Volcano  in  the 
northwestern  part  of  the  Vulsinian  District,  flowed  north  and  ended  at  the  Paglia 
River,  near  Proceno  and  Acquapendente.  The  specimen  analyzed  came  from  below 
the  castle  of  Proceno,  but  specimens  of  the  same  type  were  obtained  by  me  from 
elsewhere  in  the  flow.  From  the  descriptions  of  Bucca  and  Klein  it  is  probable 
that  the  same  type,  or  similar  ones,  occur  at  Gradoli,  Monte  Calveglia,  Poggio  Pi- 
lato,  and  elsewhere  around  the  Latera  Volcano,  though  I  can  not  confirm  this  supposi- 
tion definitely  by  actual  specimens,  as  I  was  prevented  from  visiting  these  localities. 
The  same  type  was  also  found  by  me  at  Monte  Bisenzo,  on  the  southwest  shore  of 
Lake  Bolsena,  though  the  majority  of  the  rocks  of  this  locality  belong  to  other  types 
and  magmas. 

Name. — The  derivations  of  the  subrang  names  have  been  explained  elsewhere, 
and  that  of  the  type  is  derived  from  the  Paglia  River,  at  which  the  flow  composed 
of  the  type  ends — the  most  prominent  geographical  feature  of  the  neighborhood. 

The  rocks  of  this  type  are  nearly  always  referred  to  as  leucite-tephrite  by  the 
Italian  and  German  petrographers,  on  account  of  their  content  in  labradorite.  Such 
a  designation,  however,  exaggerates  the  importance  of  this  constituent  at  the 
expense  of  the  very  much  more  abundant  orthoclase.  Furthermore,  these  rocks, 
with  their  large  leucite  phenocrysts  and  rather  light-gray  groundmass  and  abundant 


46  THE  ROMAN  COMAGMATIC  REGION. 

orthoclase,  differ  widely  in  texture  and  mode  from  the  other  rocks  of  the  region  to 
which  the  name  tephrite  is  applied,  and  which  contain  abundant  labradorite,  prac- 
tically no  orthoclase,  few  and  small  leucite  phenocrysts,  and  with  a  dark-gray 
groundmass.  For  these  reasons  the  name  of  leucite-trachyte  is  the  more  appropriate. 

PAQLIAL  PROCENOSEPULAS-KOSE.    I.    6-5.  2.  3. 

Megascopic  characters. — Light  gray,  compact,  porphyritic.  Leucite  phenocrysts  not  very 
abundant,  5  to  20  mm.  in  diameter,  fragmentary,  and  with  crystal  planes,  not  many  inclusions. 
Feldspar  phenocrysts,  few,  some  10  to  20  mm.  long,  stout  prismatic,  mostly  small  and  inconspic- 
uous; augite  phenocrysts,  very  few,  very  small,  prismatic,  black.  Groundmass :  medium  gray, 
aphanitic. 

Microscopic  characters. — Holocrystalline,  megaporphyritic,  megaphyric  and  microphyric, 
dopatic.  Megaphenocrysts:  about  15  per  cent,  leucite,  labradorite,  orthoclase,  augite.  Micro- 
phenocrysts:  about  6  per  cent,  leucite.  Groundmass:  about  80  per  cent,  trachytic  fabric; 
orthoclase,  labradorite,  augite,  nephelite,  magnetite. 

Orthoclase,  OrsAb3. — Phenocrysts:  about  i  per  cent,  10  to  20  mm.,  subhedral,  stout  pris- 
matic. Groundmass:  about  50  per  cent,  0.05  to  0.20  mm.,  anhedral,  prismatic,  arrangement 
subparallel. 

Labradorite,  about  AbiAn2. — Phenocrysts:  3  per  cent,  glomerophyric,  clusters  of  anhedral, 
stout  prisms,  clusters  0.5  to  2.0 mm.  in  diameter,  commonly  twinned.  Groundmass:  about 
15  per  cent,  0.05  to  0.20  mm.,  anhedral,  prismatic,  arrangement  subparallel. 

Leucite. — Megaphenocrysts:  10  per  cent,  5  to  20  mm.,  subhedral,  mostly  fragmentary, 
equant  to  irregular,  few  inclusions,  of  augite  and  labradorite.  Microphenocrysts:  about  6 
per  cent,  o.i  to  0.2  mm.,  anhedral,  equant,  inclusions  rare. 

Augite. — Phenocrysts:  about  2  per  cent,  0.5  to  i.omm.,  anhedral  to  subhedral,  pris- 
matic, pale  greenish-gray,  inclusions  few.  Groundmass:  about  4  per  cent,  0.02  to  0.05  mm., 
anhedral,  prismatic,  colorless. 

Nephelite. — Groundmass:  about  6  per  cent,  anhedral,  interstitial  cement. 

Magnetite. — Groundmass:  about  2  per  cent,  o.oi  to  0.02  mm.,  anhedral,  equant. 

Analysis  and  norm  as  on  p.  43. 

Type  specimen  from  below  castle,  Proceno,  northwest  of  Lake  Bolsena,  Vulsinian  District. 

I.  6.  1.  3.    Sabatinal  Beemerose  [Leucite-Phonolite,  Sabatino  Type]. 

Megascopic  characters. — Rocks  of  this  type  are  very  light  gray  and  markedly 
porphyritic,  showing  rather  numerous  large  phenocrysts  of  leucite,  with  many 
minute  ones  of  augite.  The  leucite  phenocrysts,  which  vary  in  diameter  from  2  to 
10  mm.,  are  usually  anhedral  and  with  irregular  outlines,  though  they  sometimes 
show  crystal  planes.  In  the  type  specimen  they  are  slightly  yellowish,  but  they  may 
be  normally  considered  as  colorless.  The  small  augites  are  stoutly  prismatic,  and 
very  dark  green  or  black.  The  ash-gray  groundmass  is  quite  aphanitic. 

Microscopic  characters. — In  thin  section  the  large  leucite  phenocrysts  show  the 
twinned  structure  very  well.  The  augite  phenocrysts  are  subhedral,  in  stout  prisms, 
of  a  rather  deep  olive-green,  and  with  the  optical  characters  of  aegirite-augite. 

The  groundmass  is  holocrystalline  and  shows  a  confused  fabric,  due  to  the 
diverse  arrangement  of  the  short,  stout  prisms  of  alkali-feldspar  of  which  it  is  very 
largely  composed.  These  are  either  twinned  according  to  the  Carlsbad  law,  or  more 
often  simple  crystals,  and  lime-soda  feldspar  seems  to  be  entirely  wanting.  There 
are  also  many  small,  round  anhedra  of  leucite,  which  are  not  infrequently  changed  to 


PETROGRAPHY. 


47 


the  so-called  pseudoleucite,  a  mixture  of  orthoclase  and  nephelite.  Small,  mostly 
colorless  haiiynes  are  seen  here  and  there,  with  the  customary  dusty  inclusions. 
Small  prismatic  anhedra  of  aegirite-augite  and  grains  of  magnetite  are  present  in 
very  small  amount,  and  crystals  of  apatite  and  titanite  are  still  more  rare.  Acting 
as  a  cement  for  these  is  some  nephelite,  which  is  the  last  product  of  crystallization. 
Chemical  composition. — An  analysis  of  this  type,  published  some  years  ago  in 
incomplete  form,  is  given  in  I  below,  with  a  number  of  additional  determinations. 
Two  of  the  few  other  analyses  of  beemerose  rocks  are  also  presented  in  II  and  III. 

Chemical  Composition  of  Sabatinal  Beemerose  [Leucite-phonolile]. 


I. 

II. 

III. 

SiO, 

SS-8?    o 

931 

SS.7S 

s*.  s6 

A12O3    

20.85 

204 

27.  74 

24.47 

Fe-O, 

2.  34 

oiS 

0.67 

2.  IO 

FeO    

I.  IO 

ois 

1  .  26 

I.  22 

MffO 

0.48 

OI2 

0.81 

O.  71 

CaO          .          

7.07 

oss 

0.67 

1  .  24 

Na2O                   

4.81 

077 

e,  20 

6.48 

K2O  

10.49 

112 

IO.OS 

o.  so 

H2O+      

O.  34 

I.  12 

O.O7 

H2O—    

0.7.8 

CO2  

none 

o.os 

TiOj  

0.79 

.010 

trace 

I 

ZrOj   

0.07 

0.06 

P,Oc 

O.  II 

.001 

o.  06 

SO3      

o.  14 

.002 

0.07 

Cl  

trace 

MnO  

n.  d. 

n.  d. 

BaO  

O.CQ 

trace 

IOO.SS 

99-57 

99.96 

So  er.   . 

2.  SSI 

I.  Sabatinal  beemerose   [leucite-phonolite].    Poggio  Muratella,  Lake  Bracciano,  Sabatinian 

District.     Washington,  analyst.     Jour.  GeoL,  V,  1897,  p.  49. 
II.  Grano-beemerose  [nephelite-syenite].     Itschan,  East  Cape,  Siberia.     Washington,  analyst. 

Am.  Jour.  Sci.,  XIII,  1902,  p.  176. 

III.  Grano-beemerose  [nephelite-syenite].     Beemerville,  Sussex  County,  New  Jersey.     Eakins, 
analyst.     Iddings,  Bull.  U.  S.  Geol.  Surv.  No.  150,  1898,  p.  211. 


Or  

Norm  of  I. 
.  .62.  27 

Ab    .    .    . 

.  7.67 

70.67  } 

An  

•*     ' 
.  4.7? 

Ne  

.  .  IO.  72 

,    ) 

Th  

0.28 

19.60  ' 

Di  . 

.     2.  SO 

Wo 

3  .02 

5.61 

Ol  

.   o.oo 

Mt  

X.I6 

11...    . 

1  .52 

•     4.  12 

Hm 

.     I  -44 

Rest 

IOO.OO 

0.61 

Class. 


Ratios  of  I. 
Sal 


90.27 


9-73 


Order.. 


Rang. 


Subrang. . 


'Fern 

F 
'L 

K,O'+Na,O' 
CaO' 


'Na,O' 


9.27 


=12.60 


i-45 


100.61 


48  THE  ROMAN  COMAGMATIC  REGION. 

As  regards  class,  sabatinal  beemerose  lies  toward  the  dosalanes,  but  is  far  from 
being  transitional.  In  order  and  rang  it  is  quite  close  to  the  center  points,  though 
the  amount  of  lime  is  somewhat  high.  In  the  subrang  it  approaches  the  dopotassic 
border,  though  it  can  hardly  be  called  transitional. 

Analyses  of  two  other  types  of  beemerose  are  given  in  II  and  III.  In  both  of 
these  the  mode  and  texture  differ  widely  from  the  Italian  type.  Though  they 
resemble  each  other  in  being  holocrystalline,  coarse-grained,  and  more  or  less  homo- 
metric,  they  not  only  belong  to  different  types,  but  exhibit  very  distinct  modes.  Their 
analyses  are  of  interest  in  this  connection,  not  only  as  they  represent  a  subrang  the 
rocks  of  which  are  decidedly  rare,  but  still  more  because  these  three  form  another 
excellent  illustration  of  wide  divergence  in  mode  with  great  similarity  in  chemical 
composition.  Comparison  of  the  analyses  of  these  three  rocks  and  consideration 
of  the  varying  modes  will  repay  some  study,  but  would  lead  us  too  far  astray. 

Mode. — The  mode  was  determined  by  measuring  the  leucite  megaphenocrysts 
on  the  surfaces  of  a  large  hand  specimen  and  by  measuring  the  microphenocrysts 
and  the  constituents  of  the  groundmass  at  two  magnifications.  It  can  not  be  checked 
satisfactorily  by  calculation  from  the  norm,  as  two  feldspars,  orthoclase  and  albite, 
and  the  two  lenads,  leucite  and  nephelite,  are  present,  and  the  exact  amounts  of  these 
last  can  not  be  measured,  owing  to  the  texture.  To  distribute  the  silica,  therefore, 
we  would  have  four  unknown  quantities  and  but  three  equations.  The  amount  of 
nephelite,  and  consequently  that  of  orthoclase,  as  the  two  were  measured  together,  are 
but  rough  approximations,  but  they  can  not  be  far  from  the  truth.  The  calculated 
composition  of  the  feldspar  is  that  indicated  by  the  molecules  of  potash  and  soda  left 
over  after  using  what  were  needed  for  leucite,  nephelite,  haiiyne,  and  some  acmite. 
The  amount  of  haiiyne,  calculated  as  noselite  from  the  SO3,  is  1.13,  so  that  in  this 
case,  as  in  the  preceding,  the  estimation  of  this  mineral  is  very  satisfactory.  The 
exact  amount  of  segirite-augite  can  not  be  calculated,  as  its  composition  is  unknown, 
though  an  approximation  maybe  arrived  at  by  making  assumptions  as  to  its  chemical 
composition.  If  the  acmite  present  is  calculated  on  the  basis  of  the  amount  of  norma- 
tive hematite,  the  total  pyroxene  would  be  about  10  per  cent,  which  is  undoubtedly 
much  too  high.  The  amounts  of  augite  and  magnetite  as  given  in  the  mode  are 
somewhat  too  high,  owing  to  overlapping,  but  are  probably  not  far  wrong. 

Vol.  %.  Sp.gr.  Wt.%. 

Soda-orthoclase,  Or3Ab2 71.0  X  2.6  =  184.6  68.5 

Leucite 14.4  X   2.5    =  36.0  13.4 

Nephelite 4.9  X  2.5   =  12.7  4.7 

Haiiyne i  X   2.4    =  2.6  i.o 

^Egirite-augite 6.2  X  3.4   =  21.1  7.8 

Magnetite 2.4  X  5.2   =  12.5  4.6 

100.0  269.5     100.0 

Comparing  the  mode  with  the  norm,  it  is  clear  that  the  chief  and  only  impor- 
tant points  of  difference  are  in  the  modal  presence  of  leucite,  this  being  accom- 
panied by  decrease  in  the  amounts  of  orthoclase  and  nephelite  as  compared  with 
the  norm.  The  type  therefore  may  be  described  as  leucite-salphyro-beemerose. 


PETROGRAPHY.  49 

Occurrence. — This  type  is  found  only  in  the  Sabatinian  District,  so  far  as  is 
known  with  certainty.  The  most  prominent  locality  is  a  massive  flow  on  the  north- 
west shore  of  Lake  Bracciano,  below  Poggio  Muratella,  the  lava  being  derived, 
according  to  Moderni,  from  the  cone  of  Poggio  Tondo  to  the  northwest.  The  type 
also  forms  blocks  in  bedded  tuffs  in  the  railroad  cut  about  2  km.  southeast  of  Oriolo 
station,  which  are  probably  from  the  same  cone.  Another  excellent  locality  is  on 
the  north  shore  of  Lake  Bracciano,  just  west  of  Vicarello,  at  the  end  of  a  large  lava 
stream  from  the  cone  of  Monte  Levo. 

A  rock  which  may  be  provisionally  referred  to  this  type  occurs  near  Acqua- 
pendente  (especially  at  a  quarry  at  the  east  gate),  north  of  Lake  Bolsena,  in  the 
Vulsinian  District.  No  analysis  has  yet  been  made  of  this,  and  although  it  is 
certain  that  it  falls  in  the  same  class,  order,  and  rang  as  beemerose,  it  is  possibly  in 
the  dopotassic  subrang,  which  is  as  yet  unrepresented.  From  the  descriptions  of 
Bucca  and  Klein  it  is  possible  that  rocks  belonging  to  the  same  subrang,  or  that  of 
beemerose  and  of  the  same  or  similar  type,  occur  elsewhere  in  the  district,  especially 
in  the  northwest,  around  the  Latera  Volcano. 

Name. — The  name  of  the  subrang  is  derived  from  the  locality,  Beemerville, 
near  which  a  well-known  type  occurs.  That  of  the  type  is  derived  from  that  of 
the  district  where  it  occurs. 

In  the  prevailing  systems  of  classification  these  rocks  would  be  called  either 
leucite-trachyte  or  leucite-phonolite,  according  to  Zirkel,  or  leucite-phonolite  or  leu- 
citophyre,  according  to  Rosenbusch,  as  one  does  not  or  does  lay  stress  on  the  pres- 
ence of  nephelite.  In  a  previous  paper  I  used  the  name  leucite-phonolite  in  the  sense 
of  Zirkel,  as  the  presence  of  nephelite  and  haiiyne,  even  in  rather  small  amount, 
seemed  to  be  of  sufficient  importance  to  distinguish  these  rocks  from  other  similar 
ones  of  the  region  which  are  free  from  these  lenic  minerals. 

SABATINAL  BEEMEROSE.    I.  6.  1.  3. 

Megascopic  characters. — Very  light  gray,  compact,  porphyritic.  Leucite  phenocrysts 
common,  not  well  shaped,  from  2  to  10  mm.,  often  yellowish  and  waxy  luster,  very  few  inclu- 
sions. Some  augite  phenocrysts,  about  i  mm.  long,  black.  Groundmass:  light  gray,  aphanitic. 
Specific  gravity  2.551  at  10°  C. 

Microscopic  characters. — Holocrystalline,  megaporphyritic,  dopatic.  Phenocrysts:  about 
12  per  cent,  leucite,  aegirite-augite.  Groundmass:  about  88  per  cent,  confused,  hypautomorphic 
granular  fabric,  soda-orthoclase,  leucite,  haiiyne,  nephelite,  aegirite-augite,  magnetite,  apatite, 
titanite. 

Soda-orthoclase,  Or3Ab2. — Phenocrysts:  none.  Groundmass:  about  70  per  cent,  0.2  to 
o .  4  mm.,  subhedral,  stout  prisms,  arrangement  diverse. 

Leucite. — Phenocrysts:  about  to  per  cent,  2  to  10  mm.,  subhedral  to  anhedral,  equant  to 
irregular,  twinning  common,  inclusions  of  augite  and  groundmass  rare.  Groundmass:  about  5 
per  cent,  0.05  to  o.  10  mm.,  anhedral,  equant,  few  inclusions,  sometimes  altered  to  pseudoleucite. 

jEgirite-augite. — Phenocrysts:  about  2  per  cent,  0.5  to  i.omm.,  subhedral,  stout  pris- 
matic, olive-green,  pleochroic.  Groundmass:  about  5  per  cent,  0.02  to  0.05  mm.,  anhedral, 
prismatic,  light  olive-green. 

Nephelite. — Groundmass:  about  5  per  cent,  anhedral,  cement  about  the  other  minerals 
of  the  groundmass. 


50  -THE  ROMAN  COMAGMATIC  REGION. 

Haiiyne. — Groundmass,  about  i  per  cent,  o.  i  to  0.2  mm.,  subhedral  to  anhedral,  equant 
to  irregular,  colorless  or  blue,  dusty  inclusions. 

Magnetite. — Groundmass:  about  3  per  cent,  0.02  mm.,  anhedral,  equant. 

Apatite  andtitanite. — Groundmass:  less  than  i  per  cent,  very  small,  subhedral,  usual  forms. 

Chemical  composition  as  on  p.  47. 

Type  specimen  from  below  Poggio  Muratella,  northwest  shore  of  Lake  Bracciano,  Saba- 
tinian  District. 

II-I.  7. 1.  3.    Tavolatal  Janeirose-Appianose  [Leucite-Tephrite,  Tavolato  Type, 

Tavolatite]. 

Megascopic  characters. — Rocks  of  this  type  are  light  gray  and  very  coarsely 
porphyritic.  Large  white  or  very  pale-gray  leucite  phenocrysts  are  quite  abundant. 
These  vary  in  size  from  10  to  30  mm.,  are  for  the  most  part  euhedral  with  well- 
formed  crystal  planes,  though  often  in  fragments,  and  carry  few  evident  inclusions. 
Small  blue  specks  of  haiiyne  and  fewer  small  black  prisms  of  augite  are  numerous, 
and  an  occasional  yellow  grain  of  garnet  is  visible.  The  groundmass  is  light  gray 
and  quite  aphanitic. 

Microscopic  characters. — The  large  leucite  phenocrysts  offer  few  features  of 
special  interest  in  the  thin  sections.  They  show  the  usual  twinned  structure,  and 
carry  only  a  few  inclusions  of  blue  haiiyne  and  less  often  of  augite.  The  haiiynes 
are  sometimes  euhedral  in  cubes  and  dodecahedra,  but  more  often  are  "corroded" 
deeply  and  irregularly.  They  are  of  a  rather  pale  blue  color  and  are  often  very 
dusty  with  the  common  minute  inclusions,  which  are  sometimes  arranged  in  straight 
lines,  parallel  to  the  crystal  faces  and  crossing  each  other  reticulately,  and  again  are 
irregularly  scattered  through  the  crystal  and  not  zonally  arranged.  The  small 
augite  phenocrysts,  which  form  anhedral  stout  prismoids,  are  usually  of  a  deep 
brownish-green,  somewhat  pleochroic,  and  evidently  contain  some  of  the  aegirite 
molecule. 

Small,  round,  anhedral  microphenocrysts  of  leucite  are  very  abundant  in  the 
groundmass,  their  outlines  being  rather  ill-defined  in  the  surrounding  cement.  Small 
microphenocrysts  of  feldspar  are  rare,  in  the  form  of  anhedral,  stout  prismoids. 
These  are  partly  of  orthoclase  and  somewhat  less  frequently  of  soda-lime  feldspar, 
which  varies  in  composition  from  labradorite,  AbxAn^to  anorthite.  There  are  also 
a  few  small  brown  garnets,  generally  in  well-formed  crystals,  which  frequently  show 
a  zonal  structure,  and  an  occasional  small  brown  biotite  table  is  seen. 

The  micro-groundmass  in  which  these  lie  contains  many  very  small,  slender 
feldspar  prismoids,  with  a  diverse  arrangement,  and  which  are  in  part  orthoclase 
and  to  a  much  less  extent  labradorite.  With  these  are  small  prismatic  anhedra  of 
the  green  aegirite-augite.  These  are  cemented  by  a  colorless,  feebly  doubly  refract- 
ing substance,  which  is  undoubtedly  nephelite.  It  is  noteworthy  that  magnetite 
was  wholly  absent  from  the  specimen  analyzed  and  described.  Glass  is  mentioned 
by  Struever  and  others  as  replacing  the  nephelite  base,  but  none  of  the  specimens 
which  I  collected  contain  it. 


PETROGRAPHY. 


Chemical  composition. — For  the  analysis  of  tavolatal  appianose  a  whole  hand 
specimen  was  crushed  and  sampled,  so  as  to  obtain  representative  material  without 
an  undue  amount  of  groundmass.  The  analysis  is  remarkable  for  the  very  high 
potash,  which  is  exceeded  only  by  that  in  some  of  the  analyses  of  orendose  from 
Wyoming.  The  analysis  by  Aichino  of  the  same  rock  varies  widely  from  mine  in 
many  important  particulars.  It  was  probably  made  on  unrepresentative  material, 
containing  relatively  too  much  groundmass.  This,  however,  would  not  account 
for  the  inverse  ratio  of  the  alkalis  as  compared  with  I,  and  as  the  microscopic 
examination  shows  that  potash  is  present  in  greater  amount  than  the  soda  (even  in  the 
groundmass),  it  would  seem  to  be  possible  that  the  figures  for  the  alkalis  in  II  have 
been  accidentally  interchanged,  as  has  happened  in  other  cases.  The  other  discrep- 
ancies of  note  are  also  difficult  to  account  for,  and  the  analysis  can  not  be  regarded 
as  very  trustworthy. 

Chemical  Composition  of  Tavolatal  Janeirose-appianose  [Leucite-tephrite,  Ta-volatite], 


i. 

II. 

III. 

IV. 

SiO2  

CQ.  2? 

.838 

Cl  .42 

CT.  SA 

?i  .01 

A1,O,.. 

21  .41 

.  2IO 

i8.<?7 

24.27 

20.  29 

Fe,O,  . 

1.76 

.Oil 

n.d. 

I  .  II 

3.  SO 

FeO  

1.82 

.02^ 

8.47 

1  .  24 

1  .  2O 

MgO  

O.  31 

.008 

0.48 

0.08 

O.  22 

CaO  

4.48 

.080 

2.  74 

o.  71 

1.6? 

Na2O  

q.i6 

.084 

10.38 

8.62 

8.40 

K2O  

II  .  32 

.  I2O 

6.42 

8.87 

0.81 

H2O+  

O.62  ) 

1  .00 

o.oo 

H2O—  

f 
O.  34  1 

0.88 

0.  14 

o.  10 

CO2  

O.  32 

o.  20 

O.2? 

TiO2  

O.  C7 

.OO7 

O.2O 

ZrO2  

O.O2 

P2Oc.. 

O.  12 

.OOI 

o.  14 

0.06 

SO,  . 

O.O? 

.OI3 

o.  73 

0.67 

Cl  .  .  .   . 

0.18 

.Oo6 

o.  70 

MnO  

n.d. 

trace 

n.d. 

trace 

BaO  

O.  13 

o.oo 

SrO  

trace 

0.07 

O-C1  

99.86 

0.05 

100.23 

99.87 

*ioo-58 
0.27 

99.81 

100.31 

So.  err.  . 

2  .40 

*Includes  also  0.27  F  and  trace  of  LiaO. 

I.  Tavolatal  janeirose-appianose  [hauynitic  leucite-tephrite].      Osteria  di  Tavolato,  Appian 

Way,  Latian  District.     Washington,  analyst. 

II.  Tavolatal  appianose  [haiiynitic  leucite-tephrite].    Same  locality.  Aichino,  analyst.  Sabatini, 
Mem.  Descr.  Cart.  Geol.  Ital.,  X,  1900,  p.  164. 

III.  Grano-appianose  [foyaite].     J.  M.  Henry  No.  2,  near  Chamberlain  Creek,  Magnet  Cove, 

Arkansas.     Washington,  analyst.     Jour.  Geol.,  IX,  1901,  p.  667. 

IV.  Beaveral  janeirose   [leucite-tinguaite].      Beaver   Creek,    Bearpaw  Mountains,   Montana. 

Stokes,  analyst.     Weed  and  Pirsson,  Am.  Jour.  Sci.,  II,  1896,  p.  196. 


THE  ROMAN  COMAGMATIC  REGION. 


Norm  of  I. 


Or  

„ 

O2  ) 

An  

g 

12(36.14 

Lc  

28 

78)      , 

Ne  

•    10 

L  $48.09 

HI  

Th  

i 

35  |    2.20 

Di  

Wo  

>    8.10 

64.  \ 

Mt  

2 

ss  > 

11  

I 

06   3'61 

Ap... 

O 

IA         o.  34. 

Rest  

98 
I 

48 

A1! 

Class 


Ratios  of  I. 

Sal 

Fern 


86.43 


12.05 


Order. 


Rang 


F 
'L 

K2O;+  Na,Oy 
CaO' 


Subrang. 


'Na,O' 


=  0.72 
=  9.28 
=  i-43 


99.91 

The  magmatic  position  of  the  tavolatal  appianose  is  distinctly  transitional  as 
regards  class,  the  ratio  of  sal  to  fern  being  almost  exactly  on  the  border  between  per- 
salane  and  dosalane.  The  magma  consequently  is  a  janeirose-appianose.  The 
similarity  to  an  analysis  of  a  typical  janeirose  rock  is  shown  by  comparison  of  I  and 
IV,  the  chief  divergencies  being  in  the  alkalis.  It  will  be  seen  on  reference  to  the 
norms  of  the  two  that  the  dosalic  position  of  the  latter  rock  is  brought  about  by  the 
presence  of  acmite,  both  normative  and  modal,  while  in  the  Italian  rock  this  mineral 
does  not  occur  normatively,  and  only  to  a  subordinate  extent  modally.  As  regards 
order,  rang,  and  subrang,  it  need  only  be  said  that  the  tavolatal  appianose  is  well 
within  the  respective  limits  of  these  divisions. 

The  only  other  analysis  yet  known  of  an  appianose  rock  is  one  from  Magnet 
Cove,  shown  in  III,  which  is  non-leucitic,  the  only  lenad  present  being  nephelite. 
As  the  alkalis  show,  this  latter  rock  is  quite  close  to  the  dosodic  subrang  laugenose. 

Mode. — Owing  to  the  small  size  and  confused  arrangement  of  the  groundmass 
constituents  and  the  presence  of  the  ill-defined  cement,  an  exact  estimation  of  the 
mode  by  Rosiwal's  method  was  almost  impossible,  though  it  was  practicable  for 
some  of  the  constituents,  as  the  phenocrysts  of  leucite,  labradorite,  haiiyne,  aegirite- 
augite,  and  melanite.  The  complexity  of  the  mineral  composition  also  precluded 
an  exact  calculation  from  the  norm.  By  assuming  that  the  composition  of  the 
pyroxene  was  that  of  the  aegirite-augite  of  Elfdalen,  and  that  those  of  the  haiiyne 
and  melanite  were  represented  by  well-known  analyses  of  these  minerals  from  the 
Alban  Hills,  and  checking  the  calculations  by  the  microscopic  observations,  the 
following  mode  was  estimated.  That  it  is  approximately  correct  is  shown  by  a  com- 
parison of  the  chemical  composition  recalculated  from  this  with  that  obtained  by 
chemical  analysis.  The  agreement  is  seen  to  be  as  satisfactory  as  could  have  been 
expected  when  dealing  with  such  a  complex  rock. 


Orthoclase 19.2 

Labradorite,  Abx  Ana 7.0 

Leucite 37«° 

Nephelite 11.7 

Haiiyne 10.0 


^Egirite-augite 11.5 

Melanite 2.0 

Biotite 1.3 

Apatite 0.3 


PETROGRAPHY. 


53 


I. 

II. 

I. 

II. 

SiO2  

ti.o 

so.  * 

KaO  

ii  8 

A1,O,  . 

21.  7 

21  .4. 

TiOa  

IT-3 

n   f\ 

Fe,O7 

2.4. 

1.8 

P2O<  . 

FeO  

O.  7 

1.8 

SO,  

1  .  2 

MeO    . 

0.8 

O.  7 

Cl  

CaO 

4e 

Na,O  ... 

C.  T 

C    2 

IOO.  I 

98.6 

I.  Calculated  from  the  mode.     II.  Determined  by  chemical  analysis. 

Comparison  of  this  mode  with  the  norm  shows  that,  while  the  divergencies  are 
considerable,  the  readjustments  affect  all  of  the  normative  molecules  and  are  well 
distributed.  The  readjustments  of  most  moment  are  the  formation  of  the  haiiyne 
from  normative  nephelite,  brought  about  by  the  normative  thenardite  and  halite, 
and  the  large  increase  in  the  amount  of  leucite,  chiefly  brought  about  by  the  change 
of  some  of  the  normative  nephelite  to  modal  albite  in  the  labradorite.  Readjust- 
ments of  subsidiary  importance  are  those  involved  in  the  formation  of  the  aegirite- 
augite  and  melanite,  the  latter  being  the  most  striking  varietal  mineral,  as  the  pres- 
ence of  some  modal  acmite  molecules  are  to  be  expected  in  sodipotassic  rocks  of  this 
region.  Taking  all  these  into  consideration,  as  well  as  the  fact  that  the  pheno- 
crysts  are  of  the  lenads  leucite  and  haiiyne,  we  may  describe  the  Tavolato  blocks  as 
melanitic  haiiyne-lenphyro-janeirose-appianose,  a  rather  ungainly  nomenclatorial 
product,  which  the  shorter  tavolatal  appianose  replaces  advantageously. 

Occurrence. — This  type  is  represented  only  in  the  Latian  District,  and  here  but 
sparingly.  The  best-known  locality  is  that  near  the  Osteria  di  Tavolato,  on  the 
Via  Appia  Nuova,  about  5  km.  from  Rome,  where  it  is  found  as  blocks  in  an  agglom- 
erate of  volcanic  material,  along  with  rocks  belonging  to  the  types  and  subrangs 
which  are  more  common  in  the  district.  The  provenience  of  these  Tavolato  blocks 
is,  as  yet,  uncertain,  although  they  are  undoubtedly  derived  from  the  Albano  Vol- 
cano. Closely  similar  types  have  been  described  by  Struever  and  Sabatini  from 
elsewhere  in  the  district,  as  near  the  Lago  di  Nemi. 

Name. — The  name  of  the  subrang  is  derived  from  that  of  the  old  Roman  high- 
way, the  Via  Appia,  near  which  the  type  occurs.  The  type  adjective  is  derived 
from  the  oldest  and  best-known  locality  named  above. 

In  the  prevailing  classifications  this  type  is  constantly  called  a  leucite-tephrite, 
of  the  phonolithoid  Typus  according  to  Rosenbusch.  But  the  classification  of  these 
rocks  as  tephrites  is  scarcely  justified  by  the  mode  described  above,  by  which  it  is  seen 
that  the  rock  contains  only  about  7  per  cent  of  plagioclase,  but  about  19  of  orthoclase 
and  12  of  nephelite,  and  this  subordinate  position  of  the  lime-soda  feldspar  is  more 
or  less  clearly  evident  in  the  descriptions  published  by  others.  Disregarding  this 
mineral,  then,  and  taking  into  consideration  the  very  notable  amounts  of  the  lenads 
nephelite  and  haiiyne,  the  type  should  be  called  a  haiiynitic  leucite-phonolite  in  the 
sense  of  Zirkel,  or  a  leucitophyr  of  Rosenbusch.  The  remarkable  chemical  com- 


54  THE  ROMAN  COMAGMATIC  REGION. 

position  of  this  type,  its  peculiar  mode  or  mineral  composition,  the  abundance  of 
haiiyne,  and  the  very  large  size  of  the  leucite  phenocrysts  distinguish  it  clearly  from 
nearly  all  other  leucite-phonolites,  and  in  the  prevailing  classifications  it  would  seem 
to  be  deserving  of  a  distinct  name,  for  which  that  of  tavolatite  might  be  chosen. 

TAVOLATAL  JANEIROSE-APPIANOSE.    II-I.  7.  1.  3. 

Megascopic  characters. — Light  gray,  compact,  highly  porphyritic.  Leucite  phenocrysts 
numerous,  large,  10  to  30  mm.,  well  crystallized,  but  often  fragmentary,  few  inclusions.  Hauyne 
phenocrysts  numerous  but  very  small,  up  to  i  mm.,  blue.  Augite  phenocrysts  few  and  small, 
black.  Garnet  phenocrysts  very  rare  and  small,  yellow.  Groundmass:  light  gray,  phanero- 
crystalline,  very  fine-grained. 

Microscopic  characters. — Holocrystalline,  porphyritic,  dopatic.  Megaphenocrysts:  about  30 
per  cent,  leucite,  haiiyne,  aegirite-augite,  garnet.  Microphenocrysts:  about  55  per  cent,  leucite, 
orthoclase,  haiiyne,  labradorite,  segirite-augite,  garnet.  Microgroundmass,  about  15  per  cent, 
nephelite,  orthoclase  ? 

Leucite. — Megaphenocrysts:  about  15  per  cent,  10  to  30  mm.,  euhedral  to  subhedral, 
often  fragmentary,  equant,  twinned,  inclusions  few,  of  haiiyne  and  augite.  Microphenocrysts: 
about  20  per  cent,  o .  i  to  o .  3  mm.,  anhedral,  equant  and  irregular,  often  not  well  defined,  few 
inclusions. 

Orthoclase. — Microphenocrysts,  about  20  per  cent,  o .  05  to  0.50  nun.,  anhedral  to  sub- 
hedral, stout  prismatic,  arrangement  diverse.  Microgroundmass:  about  5  per  cent  possibly 
present  as  cement. 

Nephelite. — Groundmass:  about  12  per  cent,  interstitial  cement. 

Hauyne. — Megaphenocrysts:  about  8  per  cent,  0.5  to  i.omm.,  euhedral  and  anhedral, 
equant  and  irregular,  often  deeply  corroded,  usually  blue,  many  dusty  inclusions.  Microphen- 
ocrysts: about  5  per  cent,  0.05  to  0.5  mm.,  same  characters  as  megaphenocrysts  except  size. 

Labradorite,  AbjAnz. — Phenocrysts:  about  7  percent,  0.05  to  0.50  mm.,  anhedral,  stout 
prismatic,  multiple  twinning  constant. 

jEgirite-augite. — Megaphenocrysts:  about  5  per  cent;  microphenocrysts:  about  6  per 
cent;  o .  05  to  2.0  mm.,  anhedral  to  subhedral,  stout  prismatic,  brownish-green,  pleochroic. 

Garnet. — Phenocrysts:  about  2  per  cent,  0.2  to  2.0  mm.,  subhedral,  equant,  brown,  often 
zonal. 

Biotite.  —Phenocrysts:   about  t  per  cent,  subhedral,  tabular,  brown. 

Chemical  composition,  norm,  and  mode  as  above. 

Type  specimen  from  Osteria  di  Tavolato,  Via  Appia  Nuova,  Latian  District. 

II.  4.  3.  3.    Sorianal  Har/ose  [ Biotite- Latite,  Soriano  Type]. 

The  rocks  of  this  type  occur  in  the  Roman  Region  in  two  distinct  forms,  the 
one  a  compact  lava,  the  other  a  flow-breccia.  Although  the  two  seem,  at  first  sight, 
to  be  quite  distinct,  they  may  both  be  considered  to  be  of  the  same  petrographic 
type,  the  differences  between  them  being  differences  of  consolidation  merely,  due 
to  the  viscosity  of  the  material  and  slightly  varying  accidental  conditions  during 
extrusion.  For  this  reason  they  will  be  considered  together,  though  the  field  and 
microscopic  characters  of  each  will  be  described  separately. 

Megascopic  characters — Lava  form :  These  rocks  are  very  dark,  compact  and 
highly  porphyritic.  Small  tabular  phenocrysts  of  feldspar  are  abundant,  making 
up  about  one-quarter  of  the  rock  volume.  With  them  are  many,  but  less  numer- 
ous, small,  glistening,  dark  tables  of  biotite  and  prisms  of  pyroxene.  These  lie 


PETROGRAPHY.  55 

scattered  through  a  very  dark,  sometimes  almost  black,  entirely  aphanitic  ground- 
mass,  which  may  appear  hyaline  at  times. 

Breccia  form :  This  form  of  the  type  is  much  lighter  and  somewhat  variable 
in  color,  light  gray  in  the  freshest  specimens,  and  yellowish  to  reddish  in  those  which 
are  weathered.  The  hand  specimens  usually  show  a  somewhat  streaked  or  schlieric 
appearance,  due  to  lines  and  patches  of  darker  and  lighter  gray,  or  grays  and  yellows. 
In  some  places  these  streaks  have  a  generally  parallel  and  horizontal  arrangement, 
with  the  biotite  tables  mostly  horizontal,  simulating  a  sort  of  bedded  or  flow  structure. 
The  rocks  are  quite  rough  and  soft  and  rather  friable  when  first  quarried,  but  harden 
on  exposure.  They  are  highly  porphyritic,  containing  numerous  small  white  pris- 
moids  of  feldspar,  many  small  glistening  tables  of  biotite,  and  some  prisms  of  pyr- 
oxene. The  aphanitic  groundmass  is  either  gray  or  somewhat  yellowish  or  reddish. 

Microscopic  characters — Lava  form :  The  feldspar  phenocrysts  are  seen  in  thin 
section  to  be  of  both  orthoclase  and  labradorite,  the  former  being  more  numerous. 
They  are  usually  twinned  and  do  not  carry  many  inclusions.  There  are  few  small 
anhedral  or  fragmentary  prismoids  of  a  colorless  or  very  pale-gray  pryoxene,  some 
of  .which  is  augite,  but  the  greater  part  of  which  is  hypersthene.  Biotite  tables  are 
abundant,  more  so  than  would  appear  to  be  the  case  from  the  megascopic  examina- 
tion. They  are  from  i  to  2  mm.  across,  and  are  almost  uniformly  altered,  the  larger 
ones  on  the  edges  and  the  smaller  ones  throughout,  to  a  dark,  finely  granular  aggre- 
gate. The  groundmass  in  which  these  lie,  forming  about  70  per  cent  of  the  rock,  is 
a  colorless  glass,  thickly  sprinkled  with  very  minute  prismoids  of  pyroxene,  appar- 
ently hypersthene,  some  small  feldspar  laths,  but  no  magnetite  grains.  With  low 
powers  this  glass  and  its  crystals  show  evidence  of  flow  around  the  phenocrysts.  No 
quartz  was  seen  in  any  of  my  specimens. 

Breccia  form:  The  feldspar  phenocrysts  here  also  are  of  both  orthoclase  and 
labradorite,  the  former  more  abundant,  but  they  are  for  the  most  part  angular  and 
decidedly  fragmentary,  few  showing  well-defined  prismatic  or  tabular  shapes.  There 
are  the  same  phenocrysts  of  colorless  augite  and  hypersthene,  but  these  are  also  much 
more  broken  than  in  the  preceding  form.  The  numerous  biotite  tables  are  similar 
to  those  in  the  lava  and  have  suffered  much  less  deformation  and  rupture.  The 
cement  between  these  is  either  a  dusty,  rather  indeterminate  but  evidently  vitreous 
mass,  or  a  colorless  glass  which  contains  many  minute  fragments  and  anhedra  of 
the  minerals  mentioned  above.  In  either  case  the  cement  exhibits  a  well-marked 
flow  structure  around  the  larger  crystal  fragments. 

There  has  been  much  discussion  as  to  the  character  of  this  rock,  called  locally 
"peperino,"  opinions  being  divided  as  to  whether  it  is  a  tuff  or  a  lava.  In  a  previous 
paper  I  considered  it  a  tuff,  though  somewhat  doubtfully.  But  examination  of  the 
specimens  collected  since  the  earlier  publication  would  show  that  these  rocks  are, 
for  the  most  part  at  least,  not  tuffs,  but  lava-breccias  or  flow-breccias,  that  is,  lavas 
which  had  partially  crystallized  and  solidified  during  eruption,  but  which  the  still 
moving,  very  viscous  flow  shattered  and  brecciated,  this  fluid  portion  forming  on 


THE  ROMAN  COMAGMATIC  REGION. 


solidification  the  glassy  cement.  This  view  of  their  origin  is  that  held  by  Doctor 
Cross  and  Professor  Iddings,  who  very  kindly  examined  my  specimens  and  remarked 
on  their  similarity  to  the  numerous  flow-breccias  of  the  Western  States,  with  which 
they  are  very  familiar.  This  explanation  of  their  texture  agrees  with  the  observed 
occurrences  in  the  field,  where  they  appear  for  the  most  part  to  be  flows  rather  than 
bedded  tuffs,  and  yet  have  left  unaltered  the  subjacent  rocks,  and  it  harmonizes  the 
divergent  views  as  to  their  origin. 

Chemical  composition. — A  specimen  of  the  breccia  variant  was  chosen  for  anal- 
ysis, rather  than  one  of  the  lava,  as  it  is  locally  far  the  more  important.  The  analysis 
calls  for  no  special  comment,  except  that  it  indicates  that  the  rock  is  quite  fresh. 
The  magmatic  position  of  the  rock  is  not  very  close  to  the  center  point  of  any  of  the 
divisions,  as  is  evident  from  the  ratios  given,  except  in  the  case  of  the  subrang,  where 
the  alkalis  are  almost  equal.  The  analysis  by  Ricciardi  of  a  closely  similar  rock, 
though  transitional  between  the  typical  peperino  and  that  with  large  feldspars 
according  to  Mercalli,  is  given  for  comparison,  and  the  general  resemblance  is  rather 
close.  It  must  be  remarked,  however,  that  the  figures  in  Ricciardi's  analyses  are 
often  open  to  doubt  as  to  their  accuracy,  in  many  cases  demonstrably  widely 
inaccurate,  so  that  the  analysis  is  of  little  value.  For  this  reason  the  norm  was  not 
calculated. 

Chemical  Compositon  of  Sorianal  Harzose  [Biotite-latite]. 


I. 

II. 

I. 

II. 

SiOa  

59.41     0.990 
19.06       .187 
1.87       .012 
3.42       .470 

2  .  05           .  05  1 
4.09          .073 
2.58          .042 
5.29           .056 
0.64 

58.78 
16.97 

I-I3 
2.  10 
1.46 
7.27 

3-67 
4.l8 

H3O-  
CO2 

0.91 
none 

I.OO      O.OI2 

o  .  29       .  002 
n.d 
n.d 
n.d 

3.60 

n.d 
0.32 
0.44 
trace 
0.17 

ALO,.. 

Fe,O,  

TiO2 

FeO  

p,Oe 

MgO  

SO3 

CaO  

Cl 

Na2O  

MnO 

Ko 

H2O+  

100.61 

100.09 

I.  Sorianal  harzose  breccia  [biotite-latite].     La  Cava,  near  Viterbo,  Ciminian  District.     H.  S. 
Washington,  analyst. 
II.  Peperino.     Bagnaia,   near  Viterbo,   Ciminian  District.     L.   Ricciardi,  analyst.     Cf.   Mer- 
calli, Mem.  Ace.  Line.,  XX,  1903,  p.  12. 

Q  

Norm  0}  I. 
....           ii   76 

II    76 

Or  

If     1A  ^ 

Ab  

22  .  OI  [• 

71  .  ^O 

An  

.   18  «  ) 

c  

2    •?? 

2  .  7C 

Hy.  . 

8  14 

8  14 

Mt... 

2    78  ) 

11  

!    32   \ 

4.60 

Ap  ... 

.  .                    o.  67 

o  67 

Rest  

99-02 
I  .  5^ 

85.61 


Class  

Ratios  of  I. 
Sal 

Order  

Fern 
F 

"Q 

Subrans.  . 

CaO' 
K2O' 

=6.39 

=6.08 
=  1.48 

=  i-33 


100.57 


PETROGRAPHY.  57 

Mode. — As  the  mode  is  indeterminate,  through  the  presence  of  abundant  glass, 
discussion  of  it  is  uncalled  for.  Were  the  rock  holocrystalline,  however,  with  the 
same  minerals  present  as  shown  in  this  form,  its  mode  would  probably  be  roughly 
something  like  this: 

Quartz 14 

Orthoclase,  Or4Ab3 37 

Labradorite,  Ab,An2 25 

Biotite 13 

Augite  and  hypersthene 6 

Ores 5 


Occurrence. — Sorianal  harzose  was  found  only  in  the  Ciminian  District,  around 
the  Cimino  Volcano,  where  the  lava  form  occurs  as  flows  and  as  blocks  in  the  tuffs 
at  many  places.  Among  the  localities  where  I  obtained  typical  specimens  may  be 
mentioned  flows  from  Monte  Ciliano,  along  the  highway  northwest  of  Soriano,  and 
at  Le  Piaggie,  about  3  km.  southwest  of  Soriano,  as  well  as  in  the  ravine  near  Bag- 
naia,  where  it  occurs  as  blocks  (sometimes  reddened  through  weathering)  in  the 
lower  tuffs. 

The  breccia  form  of  sorianal  harzose  (peperino)  is  met  with  in  abundance 
around  the  Cimino  Volcano,  especially  on  the  west,  north,  and  northeast,  and  it 
forms  much  of  the  lower  slopes  of  the  hills.  So  abundant  is  it  that  enumeration  of 
special  localities  is  superfluous,  and  the  only  ones  that  need  be  named  are  the  quarry 
of  L.  Mercati,  at  La  Cava,  near  Viterbo,  from  which  the  analyzed  specimen  was 
obtained,  and  the  vicinity  of  Bagnaia. 

Name. — The  derivation  of  the  subrang  name  has  been  discussed  elsewhere. 
That  of  the  type  is  derived  from  the  town  of  Soriano,  near  which  it  is  very  abundant. 

In  prevailing  classifications  there  has  been  considerable  diversity  as  to  the 
name  of  these  rocks.  The  lava  form  has  been  called  a  trachyte  by  vom  Rath,  an 
andesitic  trachyte  or  trachy-andesite  by  Mercalli,  and  a  mica-andesite  by  Deecke. 
Consideration  of  the  mode,  which  shows  almost  equal  amounts  of  alkali  and  soda- 
lime  feldspar,  would  seem  to  render  Ransome's  term  "latite"  suitable,  especially  as 
the  free  silica  has  not  crystallized  out  as  quartz.  As  the  prominent  alferric  mineral 
is  biotite  it  may  be  classed,  then,  as  biotite- latite. 

The  diversity  of  opinion  as  to  the  origin  of  the  breccia  form  has  already  been 
touched  on.  The  rocks  known  locally  as  peperiiw,  and  called  by  Brocchi  necrolite, 
on  account  of  their  use  by  the  Etruscans  for  sarcophagi,  embrace  two  distinct  types. 
The  one  is  the  peperite  des  hauteurs  of  Sabatini  and  the  trachite-andesitico  a  grossi 
sanidini  of  Mercalli,  which  is  the  earliest  product  of  eruption  and  forms  the  nucleus 
of  the  Cimino  Volcano.  This  peperino  apparently  belongs  to  the  subrang  vulsinose 
and  is  sometimes  a  lava,  sometimes  a  flow-breccia,  and  sometimes  a  tuff.  The  other 
is  the  peperite  typique  of  Sabatini  and  the  trachite-andesitico  a  piccoli  jelspati  of 
Mercalli,  which  overlies  the  other,  and  which  is  apparently  earlier  than  the  flows  of 
ciminose  to  be  described  presently.  This  peperino  is  the  breccia  form  of  sorianal 
harzose,  and  might  be  called  in  the  prevailing  nomenclature  a  biotite-latite  breccia. 


5§  THE  ROMAN  COMAGMATIC  REGION. 

SORIANAL  HARZOSE.* 

Megascopic  characters. — Dark  gray,  compact,  highly  porphyritic.  Feldspar  phenocrysts 
abundant,  2  to  5  mm.,  tabular,  colorless  or  white.  Phenocrysts  of  biotite  common,  small,  glisten- 
ing, dark-brown  tables.  Phenocrysts  of  pyroxene  rare,  small  prismatic,  black.  Groundmass 
dark  gray,  aphanitic,  sometimes  vitreous  looking. 

Microscopic  characters. — Hyalocrystalline,  megaporphyritic,  mediophyric.  Phenocrysts: 
about  40  per  cent,  orthoclase,  labradorite,  biotite,  hypersthene,  augite.  Groundmass:  about 
60  per  cent,  glass  with  microlites  of  hypersthene  and  feldspar.  Flow  structure  visible. 

Orthoclase,  Or4Ab3  ?. — Phenocrysts:  about  20  per  cent,  2  to  5  mm.,  tabular  or  stout  pris- 
matic, subhedral  to  euhedral,  Carlsbad  twinning.  Groundmass :  small  indeterminable  amount* 
o.oi  to  0.03  mm.,  prismatic,  scattered  in  flow  structure  through  the  glass  base. 

Labradorite,  AbjAn^ — Phenocrysts:  about  7  per  cent.  2  to  5  mm.,  euhedral  to  subhedral, 
tabular  or  stout  prismatic,  multiply  twinned. 

Biotite. — Phenocrysts:  about  10  per  cent,  0.05  to  0.5  mm.,  subhedral,  thin  tabular, 
brown,  much  altered. 

Hypersthene. — Phenocrysts:  about  4  per  cent,  o.  i  to  i .  o  mm.,  subhedral  to  anhedral,  pris- 
matic to  irregular,  often  fragmentary,  colorless.  Groundmass:  very  small,  indeterminable  amount, 
o.oi  to  0.03  mm.,  anhedral  prismatic,  scattered  with  flow  structure  through  the  glass  base. 

Augite. — Phenocrysts:  about  2  per  cent,  o.i  to  i.omm.,  subhedral  to  anhedral,  stout 
prismatic  or  fragmentary,  colorless.  Groundmass:  augite  may  be  present  in  very  small  amount, 
as  minute  prisms  along  with  hypersthene  in  the  glass  base. 

Glass. — Nearly  60  per  cent,  colorless,  often  dusty,  contains  the  small  microlites  described 
above,  flow  structure  common. 

Chemical  composition  and  norm  as  on  p.  56.     Mode  indeterminate. 

Type  specimens  from  Monte  Ciliano,  Soriano  Volcano,  Ciminian  District.  Type  speci- 
men of  breccia  form  from  La  Cava,  near  Viterbo,  Ciminian  District. 

III.  5.  2.  2.    Arsal  Vulsinose-Ciminose  [Vulsinite,  Arso  Type]. 

Megascopic  characters. — These  rocks  are  light  gray  when  fresh,  but  weather 
to  reddish  tones,  especially  in  the  groundmass.  They  are  decidedly  porphyritic, 
the  phenocrysts  being  quite  large  and  making  up  rather  over  one-eighth  of  the  rock 
mass.  The  great  majority  of  these,  as  well  as  the  largest,  are  of  feldspar,  which  are 
mostly  tabular  parallel  to  b  (oio)  and  from  5  to  10  mm.  long.  There  are  also  few 
small,  black,  prismatic  phenocrysts  of  augite,  and  still  more  rare  small  tables  of 
biotite.  The  light-gray  groundmass  is  clearly  phanerocrystalline  and  fine-grained. 

Microscopic  characters. — In  thin  section  the  phenocrysts  of  alkali  feldspar  are 
seen  to  be  more  common  than  those  of  the  lime-soda  feldspar.  The  former  do  not 
show  usually  the  common  characteristics  of  soda-orthoclase ;  though  the  analysis 
of  the  rock  indicates  that  they  are  quite  sodic,  while  the  latter  give  extinction  angle 
corresponding  to  an  average  composition  of  AbrAn2.  Carlsbad  twinning  is  common 
among  the  former  and  the  usual  multiple  twinning  among  the  latter.  The  feld- 
spars carry  few  inclusions,  mostly  of  augite  and  glass,  and  the  orthoclase  phenocrysts 
also  inclose  occasionally  small  inclusions  of  labradorite.  The  augite  phenocrysts 
are  subhedral  to  anhedral,  in  stout  prisms  or  fragmentary,  of  the  usual  pale  gray, 
sometimes  with  a  slightly  greenish  tinge,  though  never  pleochroic.  The  thin  tables 
of  biotite  are  light  brown  and  for  the  most  part  altered  at  the  edges  to  the  com- 
mon aggregate. 

*  This  description  applies  to  the  lava  form  of  the  type,  as  the  breccia  form  was  not  readily  measured,  and  its 
brecciated  condition  also  does  not  make  a  formal  description  necessary. 


PETROGRAPHY. 


59 


The  groundmass  is  composed  in  great  part  of  small  prisms  of  feldspar,  both  of 
orthoclase  and  labradorite,  the  quantity  of  the  former  exceeding  by  several  times 
that  of  the  latter.  The  arrangement  of  these  laths  is  subparallel,  giving  rise  to  a 
well-marked  trachytic  fabric.  There  are  scattered  through  the  groundmass  small 
prismoidal  anhedra  of  colorless  augite  and  some  small  anhedral  grains  of  magnetite. 
Between  the  feldspar  laths  is  a  little  nephelite  cement.  This  is  difficult  to  detect, 
but  one  or  two  crystal  forms  were  seen,  and  its  presence  was  verified  by  treatment 
of  the  rock  powder  with  very  dilute  acid  which  furnished  some  gelatinous  silica. 

Chemical  composition. — The  only  analysis  made  of  this  type  was  published 
some  years  ago  and  is  presented  below  in  more  complete  form,  determinations  of 
TiO3  and  P2OS  having  been  made  recently.  With  it  is  given  for  comparison  an 
analysis  of  the  bolsenal  vulsinose  [vulsinite]  from  Bolsena. 

Chemical  Composition  of  Arsal  Vulsinose-ciminose  [Vulsinite]. 


I. 

II. 

I. 

II. 

SiOj  

S7  .  V2      O.CK6 

c8.o8 

H2O  +  ...  I 

JO.  $4. 

A1,O,  . 

10.07        .187 

10  .  II 

H2O-...  \ 

o-57 

31 
O.  II 

Fe,O,    . 

2.21           .014 

2.  ce 

CO,  

none 

none 

FeO       

2.  v.       .033 

1  .00 

TiO2  

0.61     0.008 

0.82 

MeO 

1  .  60          .  040 

I  .<X 

PSOr. 

0.17       .001 

o.  20 

CaO 

3  82        068 

7.  76 

MnO      

n.d. 

n.d. 

n      B  A 

K30  

9.15     .097 

8.86 

100.09 

99.92 

So.  ST.  .  . 

2  .  61  1  at  11° 

2  .  ^34  at  2%° 

I.  Arsal  vulsinose-ciminose  [vulsinite].     Near  Vetralla,  Monte  Vico,  Ciminian  District. 

ington,  analyst.     Jour.  Geol.,  IV,  1896,  p.  849. 
II.  Bolsenal  vulsinose  [vulsinite].     Bolsena,  Vulsinian  District. 


Wash- 


Norm  of  I. 
Or 53.93 


Ab  

16 

11 

L 

An  

10 

Ne  

6S 

t 

Di  

c 

on 

I 

Ol  

I 

87 

7 

86 

Mt  

) 

11  

I 

22 

4 

47 

Ap  .. 

o 

0 

u 

OT- 

OT- 

Rest  

99 
o 

61 

.">  / 

IOO 

18 

Class . 


86.94 


12.67 


Ratios  of  I. 

Sal 
'  'Fern 


Order. 


Rang. 


Subrang 


F_ 
'L 

KaO'+Na3O' 
CaO' 

K,O' 


'Na,O' 


6.86 


3-92 


1.87 


From  the  figures  given  above  it  is  clear  that  this  type  falls  well  within  the  perfelic 
order,  the  domalkalic  rang,  and  the  dopotassic  subrang.  On  the  other  hand,  it  is 
almost  exactly  on  the  border  line  of  the  persalane  class,  where  it  was  formerly*  con- 
sidered to  belong.  The  change  in  its  position  is  due  to  the  recent  determination  of 
TiO2  and  P2OS,  which  were  not  reported  in  the  original  analysis,  and  which  just 

*  H.  S.  Washington,  Prof.  Paper  U.  S.  Geol.  Surv.  No.  14,  iQ°3 1  PP-  i°8-  J99- 


6o 


THE  ROMAN  COMAGMATIC  REGION. 


add  sufficiently  to  the  femic  minerals  to  carry  the  rock  across  the  line.  This  is  one 
of  several  instances  of  the  same  kind  which  occurred  in  the  present  investigation. 

This  transitional  character  is  also  shown  by  the  very  close  correspondence 
between  the  analysis  of  the  ciminose  and  that  of  the  bolsenal  vulsinose  given  in  II. 
The  difference  in  position  is  due  partly  to  the  slightly  higher  FeO  and  MgO  of  II, 
as  well  as  to  its  lower  silica  and  higher  alkalis.  The  two  former  involved  the  pres- 
ence of  an  absolutely  greater  amount  of  normative  femic  minerals,  while  the  two 
latter  necessitated  the  formation  of  a  considerable  amount  of  nephelite  from  some 
of  the  soda,  and  thus  reduced  notably  the  relative  amount  of  salic  minerals. 

While  the  effect  of  such  a  shifting  of  the  classificatory  position  through  the 
influence  of  very  small  amounts  of  minor  chemical  constituents  may  seem  to  be  a 
serious  defect  in  the  system  of  classification,  it  is  in  fact  not  so.  For  rocks  whose 
position  can  be  thus  changed  are  transitional  in  character,  and  in  the  nomenclature 
the  change  is  simply  one  corresponding  to  the  present  case,  from  ciminose- vulsinose 
to  vulsinose-ciminose,  and  it  is  a  matter  of  very  slight  moment  in  which  of  the  two 
contiguous  divisions  the  rocks  strictly  belong. 

In  the  present  case  it  is  quite  certain  that  some  of  the  rocks  of  the  region,  which 
will  be  mentioned  later,  fall  in  vulsinose  rather  than  in  ciminose,  so  that  they  belong 
to  ciminose-vulsinose,  or  are  even  possibly  so  far  from  the  border  as  to  be  well  within 
vulsinose  itself  and  not  properly  transitional.  But  no  analyses  of  these  were  made 
and  it  was  not  thought  advisable  or  sufficiently  important  to  erect  these  at  present 
into  distinct  types. 

Mode. — The  calculation  of  the  mode  from  the  norm  is  comparatively  simple. 
For  the  augite  the  composition  of  that  from  Ticchiena  (cj.  p.  134)  was  assumed, 
for  the  small  amount  of  biotite,  a  mixture  in  equal  parts  of  leucite  and  olivine. 
After  assigning  sufficient  Na2  O  to  form  albite  in  the  proportion  toward  the  anorthite 
demanded  by  the  average  composition  AbxAn2,  the  rest  was  distributed  between 
albite  (which  enters  into  the  alkali-feldspar)  and  nephelite,  as  there  was  insufficient 
silica  to  form  the  polysilicate  molecule  with  all  of  it.  In  determining  the  mode 
by  the  microscope  it  was  found  to  be  impossible  to  distinguish  accurately  between 
the  small  laths  of  orthoclase  and  labradorite,  as  well  as  between  these  and  the 
interstitial  nephelite,  so  that  the  total  amount  of  these  is  given. 


CALCULATED. 


MEASURED. 


Orthoclase,  O^Abj  . 
Labradorite,  AbiAn2 

Nephelite 

Augite 

Biotite 

Magnetite 

Apatite 


Vol.%.       Sp.gr. 

85.4     X     2.6     =  222.0 

10.4   X  3.3   =  34-3 

2.1    X   2.9   =  6.1 

2.1     X     5-2     =  10-9 


Wt.  %. 

81.3 
12.5 

2.2 

4.0 


273.3          IOO-° 


It  is  seen  that  the  two  methods  yield  very  concordant  results  in  this  case,  the 
error  due  to  overlapping  not  being  serious.     Of  the  two  the  calculated  mode  must 


PETROGRAPHY.  61 

be  regarded  as  the  more  reliable,  both  on  the  score  of  freedom  from  this  error  and 
on  that  of  completeness. 

Compared  with  the  norm  the  only  divergence  of  note  is  that  involved  in  the 
formation  of  biotite,  which  takes  up  the  normative  olivine  (used  as  the  basis  for  cal- 
culating the  biotite)  and  a  little  of  the  orthoclase,  as  well  as  the  transfer  of  a  very 
small  portion  of  the  normative  anorthite  and  magnetite  to  form  the  augite.  But 
these  readjustments  are  of  slight  importance  and  their  effect  is  negligible,  so  that 
the  mode  may  be  considered  normative  and  the  rock  may  be  described  as  normative 
salphyro-vulsinose-ciminose. 

Occurrence. — Typical  arsal  vulsinose-ciminose  is  met  with  especially  in  the 
Ciminian  District,  to  the  west  and  southwest  of  Monte  Vico,  where  it  forms  extensive 
flows.  The  most  prominent  localities  are  in  the  neighborhood  of  Vetralla  and  San 
Giovanni  di  Bieda,  where  it  is  quarried  extensively.  It  is  also  probable  that  the 
type  occurs  in  the  northern  part  of  the  Vulsinian  District  and  possibly  in  the  Aurun- 
can,  but  of  this  there  can  be  no  certainty  in  the  absence  of  specimens  and  the  meager- 
ness  of  the  published  descriptions. 

Rocks  which  belong  rather  to  ciminose-vulsinose  or  to  vulsinose,  and  of  this 
type,  are  abundant  about  the  Cimino  Volcano  in  the  Ciminian  District,  as  at  Monte 
Pallanzana  and  Monte  Valentino,  where  they  form  sometimes  lava  flows,  some- 
times flow-breccias,  and  again  tuffs.  These  are  the  peperite  des  hauteurs  of  Sabatini 
and  the  trachite-andesitico  a  grossi  sanidini  of  Mercalli,  already  mentioned  in  connec- 
tion with  sorianal  harzose.  In  these  rocks  the  orthoclase  phenocrysts  sometimes 
attain  a  large  size,  up  to  5  cm.  in  length.  The  flow  from  the  Vico  Volcano  at  Massa 
di  San  Sisto,  southwest  of  Viterbo,  in  the  Ciminian  District,  and  flows  near  San 
Lorenzo  in  the  northern  part  of  the  Vulsinian  District,  also  seem  to  belong  in  vul- 
sinose rather  than  in  ciminose.  As  the  specimens  of  these  were  not  very  fresh  no 
analyses  of  them  were  made. 

Name. — The  subrang  name,  ciminose,  like  the  older  ciminite,  is  derived  from 
that  of  the  Ciminian  District  in  which  this  magma  is  very  common.  The  type 
name,  arsal,  is  derived  from  the  flow  of  L'Arso,  on  the  island  of  Ischia,  as  the  mode 
and  texture  and  the  habit  of  the  rocks  described  above  much  resemble  those  of  the 
arsal  monzonose  from  the  Campanian  locality.  At  the  same  time,  it  must  be  said 
that  the  Arso  rock  differs  from  those  from  the  Ciminian  and  Vulsinian  Districts  in 
the  greater  conspicuousness  of  the  feldspar  phenocrysts  as  well  as  in  the  presence 
of  accessory  olivine  phenocrysts.  On  this  account  it  might  be  well  to  distinguish 
between  them  and  erect  two  types  instead  of  one,  restricting  the  term  "arsal"  to 
rocks  identical  with  the  Arso  lava  and  calling  those  with  less  conspicuous  feldspars 
and  with  very  small  amounts  of  biotite  and  no  olivine,  vetrallal  ciminose,  vulsinose, 
or  monzonose,  as  the  case  may  be.  But  it  would  seem  unwise  at  the  present  time  to 
increase  the  already  large  number  of  types  described  here,  and  the  recognition  of 
such  rather  unimportant  differences  may  well  be  left  to  the  future  if  it  be  then  deemed 
advisable. 

As  regards  the  position  of  the  type  in  the  prevailing  systems  of  classification, 
the  remarks  in  connection  with  bolsenal  vulsinose  (p.  33)  will  apply  here.  Placed 


62  THE  ROMAN  COMAGMATIC  REGION. 

generally  in  the  trachytes,  they  are  distinguished  from  the  most  typical  of  these  by  the 
abundance  of  labradorite  and  by  their  chemical  composition,  on  account  of  which 
the  name  of  vulsinite  has  been  bestowed  on  rocks  of  this  type.  To  distinguish 
them  from  the  vulsinites  which  belong  to  the  type  of  bolsenal  vulsinose,  with  the 
much  smaller  and  less  abundant  phenocrysts,  the  present  rocks  may  be  referred  to 
the  Arso  type,  while  the  others  are  of  the  Bolsena  type. 

ARSAL  VULSINOSE-CIMINOSE.    I-II.  5.  2.  2. 

Megascopic  characters. — Light  gray,  compact,  rough,  porphyritic.  Feldspar  phenocrysts 
rather  numerous,  large,  tabular,  colorless.  Augite  phenocrysts  rare,  very  small,  prismatic,  black. 
Biotite  phenocrysts  very  rare,  thin  tabular,  brown.  Groundmass:  very  light  gray,  phanerocrys- 
talline,  very  fine-grained. 

Specific  gravity  2.611  at  11°  C. 

Microscopic  characters. — Holocrystalline,  megaphyric,  dopatic.  Phenocrysts:  about  20  per 
cent,  orthoclase,  labradorite,  augite,  biotite.  Groundmass:  about  80  per  cent,  orthoclase,  labra- 
dorite, augite,  magnetite,  nephelite.  Fabric  trachytic. 

Orthoclase. — Phenocrysts:  about  10  per  cent,  5  to  10  mm.  long,  subhedral,  tabular,  Carls- 
bad twinning  common,  inclusions  few,  of  augite  and  labradorite.  Groundmass:  about  50  per 
cent,  0.05  to  o.  20  mm.,  anhedral,  stout  prismatic  and  irregular  equant,  arrangement  subparallel. 

Labradorite,  AbxAn2. — Phenocrysts:  about  5  per  cent,  3  to  7  mm.,  subhedral,  tabular, 
much  twinned,  inclusions  few,  of  augite  and  magnetite.  Groundmass:  about  10  per  cent, 
0.05  to  o.2omm.,  anhedral,  stout  prismatic,  arrangement  subparallel. 

Augite. — Phenocrysts:  about  4  per  cent,  0.5  to  2.0  mm.,  anhedral,  stout  prismatic  and 
fragmentary,  pale  greenish-gray,  non-pleochroic.  Groundmass:  about  5  per  cent,  0.02  to 
0.06  mm.,  anhedral,  prismatic  and  equant,  colorless  or  very  pale-greenish. 

Nephelite. — Groundmass :  about  5  per  cent,  rarely  subhedral,  mostly  anhedral,  inter- 
stitial between  the  groundmass  feldspar  laths. 

Biotite. — Phenocrysts:  about  3  per  cent,  0.5  to  2.0  mm.,  subhedral,  thin  tabular,  light 
brown,  somewhat  altered  at  the  edges. 

Magnetite. — Groundmass:  about  3  per  cent,  o.oi  to  0.05  mm.,  anhedral,  equant. 

Apatite. — Groundmass:  about  0.5  per  cent,  subhedral,  prismatic. 

Chemical  composition,  norm,  and  mode  as  above. 

Type  specimen  from  near  Vetralla,  Ciminian  District. 

II.  5.  2.  2.    Piescolal  Citninose  [Ciminite,  Fiescoli  Type]. 

Megascopic  characters. — This  type  is  related  texturally  to  the  arsal  ciminose, 
just  as  is  the  bolsenal  vulsinose  to  arsal  vulsinose,  the  difference  consisting  in  the 
size  and  number  of  the  feldspar  phenocrysts.  It  differs  also  modally  from  the  arsal 
type  in  the  presence  of  olivine  instead  of  biotite,  though  this  is  of  comparative 
unimportance. 

The  rocks  are  rather  light  to  rather  dark  gray,  and  in  the  hand  specimen  show 
but  few  and  small  phenocrysts  of  feldspar,  which  are  tabular  and  not  very  con- 
spicuous. There  are  also  small  phenocrysts  of  black  augite  and  of  yellow  olivine, 
but  these  are  not  very  numerous.  The  gray  groundmass  is  quite  aphanitic,  and 
most  of  the  specimens  are  compact,  though  a  few  are  somewhat  vesicular. 

Microscopic  characters. — The  not  very  abundant  phenocrysts  of  feldspar  are 
seen  in  the  thin  section  to  be  of  both  orthoclase  and  of  a  labradorite  of  about  the 
composition  AbjAn2.  They  are  usually  tabular,  sometimes  prismatic,  and  often 


PETROGRAPHY. 


show  some  crystal  planes,  or  are  subhedral.  Twinning  is  common,  but  is  not  very 
marked,  and  in  most  specimens  the  phenocrysts  of  both  feldspars  are  about  equal 
in  amount.  Small,  subhedral,  often  fragmentary  phenocrysts  of  augite  are  rather 
more  abundant,  and  are  almost  wholly  colorless,  only  the  larger  ones  showing  a 
faint  greenish  tinge.  The  somewhat  less  numerous  olivine  phenocrysts  are  like- 
wise small  and  subhedral,  most  of  them  having  the  customary  crystal  planes.  They 
are  much  less  fractured  than  the  augites  and  occasionally  are  euhedral.  In  thin 
section  they  are  colorless,  but  often  have  a  thin  outer  border  of  yellow  or  brown 
substance,  which  is  probably  iddingsite. 

The  groundmass  is  typically  holocrystalline  and  the  fabric  a  felted  one,  due  to 
the  diverse  arrangement  of  the  feldspar  laths.  It  is  made  up  in  great  part  of  very 
small  laths  of  feldspar,  those  of  orthoclase  more  numerous  than  those  of  labrador- 
ite,  with  small  prismoids  of  augite  and  grains  of  magnetite.  These  are  scattered 
without  definite  arrangement,  giving  rise  to  the  felted  fabric,  through  a  colorless 
cement,  which  usually  shows  somewhat  faint  birefringence  and  is  referred  to  ortho- 
clase. In  a  few  specimens  this  is  replaced  by  a  colorless  glass,  the  amount  of  which 
is  always  negligible,  and  it  is  often  a  matter  of  difficulty  to  decide  with  finality  on 
the  holocrystalline  character  of  such  groundmasses.  In  rare  cases,  as  in  a  flow 
south  of  Soriano,  the  feldspathic  cement  presents  an  ill-defined  poikilitic  texture, 
irregular  and  vaguely  bounded  areas  extinguishing  simultaneously. 

Chemical  composition. — Of  fiescolal  ciminose  two  analyses  are  available,  both  of 
which  have  been  published  previously.  As  given  below  they  include  recent  deter- 
minations of  P2OS  and  also  a  redetermination  of  TiO2  in  I,  with  proper  corrections 
of  the  figures  for  A12O3.  It  must  be  noted  that  the  original  analysis  of  this  type* 
was  incorrect  as  regards  alumina  and  magnesia,  as  was  pointed  out  in  a  later  pub- 
lication,! and  is  therefore  to  be  rejected  and  replaced  by  that  given  here. 

Chemical   Composition   of  Fiescolal  Ciminose  [Ciminite]. 


I. 

II. 

I. 

II. 

SiOj  

sC.4.6       O.O24. 

C7.3.I       O.QCC 

CO2.  .  . 

none 

none 

A12O3  

IJ../17           .  14.2 

I4.4.I          .141 

TiOa  . 

O.C3       O.OO7 

o  .  40    o  .  005 

Fe2O3  

1  .  74.          .  OO8 

I  .  2  I           .  OO8 

P2OS.. 

0.36       .003 

0.30       .002 

FeO  

4..  CO          .062 

4..  77          .o6l 

MnO. 

n.d. 

n.d. 

MgO  

7.00       .  108 

7.8o         .IOC 

BaO.. 

none 

f~"o/~» 

Na,0  

1.79     .029 

1.35           .022 

100.21 

100.61 

KaO  
H2O+  

6.63       .071 

O.23  ) 

6.38          .068 

Sp.  gr.  .  . 

2  .  70  at  10° 

H2O-  

u.^  r 
O.  1C  \ 

0.18 

I.  Fiescolal   ciminose   [ciminite].      Fontana    Fiescoli,    Cimino   Volcano,    Ciminian   District. 

Washington,  analyst.     Am.  Jour.  Sci.,  IX,  1900,  p.  44. 

II.  Fiescolal  ciminose  [ciminite].     La  Colonetta,  Cimino  Volcano,  Ciminian  District.     Wash- 
ington, analyst.     Am.  Jour.  Sci.,  IX,  1900,  p.  44. 


*  Jour.  Geol.,  IV,  1896,  p.  837. 


Jour.  Sci.,  IX,  1900,  p.  45. 


THE  ROMAN  COMAGMATIC  REGION. 


Norms. 


I. 


II. 
0.30 


, 

2'62 
0.67 


Q  ..........................................  none  o  .  30 

Or  ........................................  39.48)  37.81)        '    f63.8a 

Ab  ........................................  15.20  [•  66.36         11.53  ^63.52 

An  ........................................  11.68)  14.18  ) 

Di  ........................................  14.89  )  14.46  ) 

Hy  .......................................   3.59  [29.46  18.92  [33.38 

Ol  ........................................  10.98)  none    ) 

Mt  .......................................   I.86/  33.  ?o  1.86)        ,       36.65 

II  ........................................   i.o65  2"92  0.76  \ 

Ap  .......................................  0.92  0.92  0.67 

99.66  100.49 

Rest  ......................................  0.38  0.18 

100.04  100.67 

Ratios. 

I.  II. 

_.  Sal 

Class  ......................  j^n"  =I'99         J'74 

F      F 
Order  .....................  T~°ro  =°°         21I-73 

K20'  +  Na,0' 
Rang  .....................  -  ^^  -  =2.38         1.76 

K20' 
Subrang  ...................  i^Q7  =2-45         3-°9 

The  very  close  similarity  of  the  two  analyses  is  striking,  all  the  respective  figures 
being  almost  close  enough  to  be  satisfactory  duplicates,  except  in  the  case  of  silica, 
which  is  higher  by  2  per  cent  in  II  than  in  I.  In  this  connection  it  must  be  said  that 
the  specimen  of  this  rock  differs  from  the  other  in  containing  a  very  small  amount 
of  glass,  instead  of  being  holocrystalline  as  is  the  rock  from  Fontana  di  Fiescoli. 
Corresponding  to  this  chemical  difference  the  norm  of  II  shows  not  only  none  of 
the  abundant  olivine  of  I,  but  hypersthene  in  its  stead  and  even  a  trifling  exces 
of  silica  which  appears  as  normative  quartz. 

As  regards  the  classificatory  positions  of  the  rocks  little  need  be  said.  The) 
are  much  alike  and  fall  well  within  the  boundaries  of  the  several  divisions,  except 
that  II  is  rather  close  to  the  border  of  salfemane  and  might  strictly  be  regarded 
transitional,  that  is,  belonging  to  the  magma  prowersose-ciminose,  though  this 
refinement  seems  to  be  scarcely  advisable.  This  difference  in  position  is,  of  course, 
connected  with  the  presence  of  hypersthene  in  the  norm  of  II  rather  than  olivine, 
and  is  a  consequence  of  its  higher  silica. 

Mode.  —  As  one  of  the  rocks  analyzed  is  holocrystalline  and  the  other  contains 
a  small  amount  of  glass,  and  as  their  norms  also  differ  somewhat,  the  modes  of  the 
two  were  determined  separately.  On  account  of  the  very  fine-grained  charac- 
ter of  the  groundmass  and  its  felted  texture,  the  mode  could  not  be  estimated  in  its 
entirety  by  Rosiwal's  method,  though  the  amounts  of  the  phenocrysts  were  easil) 
ascertained. 

In  calculating  the  mode  of  the  Fiescoli  rock  few  readjustments  were  needed.  Th< 
normative  amount  of  olivine  was  taken  as  the  modal,  as  this  agreed  with  that  meas- 


PETROGRAPHY.  65 

ured  in  thin  section.  The  small  amount  of  normative  hypersthene  was  assumed 
to  belong  to  the  augite  molecule,  though  the  orthorhombic  pyroxene  may  actually 
exist  among  the  small  pyroxene  prisms,  since  we  find  it  in  the  sorianal  harzose  from 
the  same  volcano.  The  results  of  the  calculation  were  as  follows: 

Soda-orthoclase,  Or,0Ab3 50. 7 

Labradorite,  Abj Ana 13.1 

Augite 23 . 2 

Olivine 11.2 

Magnetite 0.9 

Apatite 0.9 


The  calculation  of  the  mode  of  the  Colonetta  rock  was  also  simple.  An 
amount  of  the  normative  hypersthene  corresponding  to  that  in  the  preceding  case 
was  assumed  to  enter  into  the  augite  molecule,  while  the  balance  was  recalculated 
as  olivine,  the  amount  of  which  agrees  fairly  well  with  the  appearance  of  the  section. 
This  calculation  of  olivine  necessarily  adds  to  the  amount  of  free  silica,  which  is 
stated  as  quartz,  and  which  we  must  assume  to  be  present  in  the  glass  base.  In 
the  case  of  both  rocks  the  composition  of  the  augite,  apart  from  the  hypersthene 
molecules,  was  assumed  to  be  that  of  the  Ticchiena  augite  referred  to  elsewhere. 
These  readjustments  give  the  following  figures: 

Soda-orthoclase,  Ore Abj 43-6 

Labradorite,  AbjAn^ 16.  i 

Quartz 4.6 

Augite 22.4 

Olivine 11.7 

Magnetite 0.9 

Apatite 0.7 


It  is  evident  that  the  two  modes  are  essentially  identical,  the  amount  of  quartz 
or  glass  being  so  small  as  to  be  negligible.  Taking  the  presence  of  olivine  into 
account  the  type  may  be  described  as  normative  olivinic  salfemphyro-ciminose;  for 
which  the  shorter  term  "fiescolal  ciminose"  is  an  advantageous  substitute. 

The  occurrence  of  quartz  or  a  highly  quaric  glass  along  with  olivine  need  not 
cause  surprise,  as  a  number  of  similar  cases  are  well  known.  Indeed,  from  the 
generally  observed  easy  solidification  of  highly  quaric  magmas  or  partial  magmas 
before  complete  crystallization,  the  presence  of  glass  in  the  specimen  which  shows 
some  normative  quartz,  while  that  without  this  is  holocrystalline,  is  readily  intelligible. 

Occurrence. — Rocks  of  this  type  are  confined  to  the  Ciminian  District,  so  far 
as  known.  In  this  they  are  very  abundant  around  the  Cimino  Volcano,  and  among 
representative  localities  may  be  mentioned  the  stream  bed  at  Cavorcie,  the  quarry 
of  the  Chiesa  dei  Frati  at  Madonna  della  Quercia,  a  flow  west  of  Villa  Lante,  flows 
and  blocks  in  the  tuffs  at  Monte  Ciliano  and  near  Soriano,  and  the  type  localities  at 
La  Colonetta  and  Fontana  di  Fiescoli. 

Name. — The  name  of  the  subrang  has  been  already  explained  as  derived,  like 
that  formerly  proposed  for  the  group  of  trachytic  rocks  characterized  by  the  presence 


66  THE  ROMAN  COMAGMATIC  REGION. 

of  both  orthoclase  and  labradorite  with  some  olivine,  from  the  name  of  the  dis- 
trict. The  type  adjective  is  based  on  the  locality,  Fontana  di  Fiescoli,  whence  the 
original  type  specimen  of  ciminite  came. 

In  the  prevailing  classifications  these  rocks  have  been  given  a  wide  diversity 
of  names,  almost  every  geologist  who  has  studied  the  district  having  chosen  a  dif- 
ferent one.  Thus  vom  Rath  calls  them  trachytes,  Deecke  augite-andesites,  in  both 
cases  accompanied  by  remarks  on  their  anomalous  mineralogical  and  chemical  char- 
acters; Bucca  speaks  of  them  as  trachytes,  Mercalli  first  called  them  oli vine-trachytes 
and  later  olivine-andesites,  while  the  name  selected  by  Sabatini  is  "labradorite," 
used  with  the  French  signification. 

In  view  of  this  confusion,  which  amounts  to  a  tacit  admission  that  the  type 
belongs  neither  to  the  trachytes  nor  to  the  andesites,  a  special  name  seems  advisable, 
and  that  formerly  selected,  "ciminite,"  is  appropriate,  as  the  rock  is  abundant  at 
Monte  Cimino  and  was  at  the  time  the  only  one  which  it  seemed  to  be  worth  while 
to  name.  For  these  reasons  Mercalli's  recent  criticism  of  the  name  "ciminite,"  on 
the  ground  that  the  rock  does  not  make  up  the  main  part  of  Monte  Cimino,  seems 
to  be  somewhat  strained. 

PIESCOLAL  CIMINOSE.    II.  5.  2.  2. 

Megascopic  characters. — Medium  gray,  compact,  somewhat  porphyritic.  Feldspar  pheno- 
crysts  few,  i  to  5  mm.,  tabular  or  prismatic.  Olivine  phenocrysts  rather  abundant,  i  to  2  mm., 
yellow.  Augite  phenocrysts  rather  abundant,  i  to  2  mm.,  stout  prismatic,  black.  Ground- 
mass:  rather  light  to  rather  dark  gray,  aphanitic. 

Specific  gravity  2.70  at  10°. 

Microscopic  characters. — Holocrystalline,  rarely  percrystalline,  mediophyric,  dopatic. 
Phenocrysts:  about  25  per  cent,  olivine,  augite,  orthoclase,  labradorite.  Groundmass:  about 
75  per  cent,  orthoclase,  labradorite,  augite,  olivine,  magnetite,  apatite,  glass  in  rare  cases. 
Groundmass,  texture  felted. 

Orthoclase,  about  O^Abj. — Phenocrysts:  about  3  per  cent,  i  to  5  mm.,  subhedral  stout 
prismatic  or  tabular,  Carlsbad  twinning  common,  inclusions  few.  Groundmass:  about  40  or 
45  per  cent,  0.02  to  o.iomm.,  partly  subhedral  to  anhedral,  slender  prismatic,  arrangement 
diverse,  partly  anhedral,  interstitial  cement. 

Labradorite,  A.bIA.n2. — Phenocrysts:  about  3  per  cent,  i  to  5  mm.,  subhedral  to  anhedral, 
stout  prismatic,  much  twinned,  often  clustered,  inclusions  few.  Groundmass:  about  10  per  cent, 
0.02  to  o.iomm.,  anhedral,  prismatic,  arrangement  diverse. 

Augite — Phenocrysts:  about  9  per  cent,  0.2  to  2.0  mm.,  subhedral  to  anhedral,  stout 
prismatic,  often  fragmentary,  colorless  or  very  pale  greenish,  few  inclusions.  Groundmass: 
about  15  per  cent,  o.oi  to  0.05  mm.,  anhedral,  slender  prismatic,  colorless. 

Olivine. — Phenocrysts:  about  10  per  cent,  0.5  to  2.0  mm.,  subhedral,  stout  prismatic, 
tabular  or  equant,  often  corroded,  colorless,  commonly  with  narrow  border  of  yellow  iddingsite. 

Magnetite. — Groundmass:  about  2  per  cent,  o.oi  to  0.02  mm.,  anhedral,  equant. 

Apatite. — Groundmass:   about  i  per  cent,  subhedral,  slender  prismatic. 

Glass. — Usually  absent:  about  5  per  cent,  colorless,  interstitial  between  feldspar  laths, 
difficult  to  detect. 

Chemical  composition  and  norm  as  above. 

Type  specimens  from  Fontana  Fiescoli  and  from  La  Colonetta,  Cimino  Volcano,  Ciminian 
District. 


PETROGRAPHY. 


II.  5.  2.  2.     Bagnorea!  Ciminose  [Leucite-Trachyte,  Bagnorea  Type]. 

Megascopic  characters. — Rocks  of  this  type  are  light  gray  in  color,  and  while 
porphyritic  are  not  strikingly  so,  as  the  phenocrysts  are  neither  numerous,  large, 
nor  conspicuous.  Those  of  leucite  are  most  abundant,  but  constitute  only  from  5 
to  10  per  cent  of  the  rock.  They  are  from  i  to  5  mm.  in  diameter  and  usually 
anhedral  to  subhedral,  crystal  planes  not  being  well  developed.  There  are  a  few 
very  small  black  prismoids  of  augite,  with  an  occasional  biotite  table.  The  ground- 
mass  is  rather  light  gray  and  quite  aphanitic. 

Microscopic  characters. — In  thin  section  the  leucite  phenocrysts  show  the  usual 
characters,  though  the  birefringence  is  weaker  than  in  most  cases  and  inclusions 
are  rare.  The  phenocrysts  of  augite  are  subhedral,  stout  prisms,  often  fragmen- 
tary, and  are  of  a  somewhat  greener  tint  than  usual,  though  not  markedly  so.  The 
biotite  tables  are  brown,  and  for  the  most  part  are  much  altered  to  the  common 
granular  aggregate.  Phenocrysts  of  feldspar  are  visible  in  the  thin  section,  which 
escape  observation  in  the  hand  specimen.  They  are  of  both  orthoclase  and  plagio- 
clase,  the  latter  a  labradorite  of  about  AbjAn,,.  Around  these  phenocrysts  mantles 
of  later  alkali  feldspar,  oriented  like  the  nuclear  crystal,  are  not  uncommon. 

The  groundmass  is  typically  holocrystalline  and  shows  an  intersertal  fabric. 
It  is  composed  in  large  part  of  feldspar  prisms,  most  of  these  being  of  orthoclase 
with  fewer  of  labradorite,  the  arrangement  being  diverse,  with  the  other  ground- 
mass  constituents  between  the  laths.  Small  round  leucites  are  quite  abundant, 
carrying  few  inclusions,  and  there  are  present  in  considerable  amount  small  color- 
less augites,  both  in  the  form  of  subhedral  prismoids  and  as  anhedral,  equant  grains. 
Small  grains  of  olivine  are  rather  rare  and  the  usual  magnetite  grains  and  apatite 
needles  are  seen.  In  some  specimens  there  is  a  residual  base  of  nephelite,  intersti- 
tial between  the  other  constituents,  but  this  is  not  always  found  and  its  amount  is 
very  small  in  any  case. 

Chemical  composition. — Incomplete  analyses  were  made  of  two  specimens  of  this 
type  some  years  ago  and  are  here  republished,  with  several  additional  determinations. 

Chemical  Composition  of  Bagnoreal  Ciminose  [Leucite-trachyte]. 


I. 

II. 

I. 

II. 

SiO  3  

CC.8?       O.Q3I 

CC.2I       O.Q2O 

TiO2. 

O.<;Q     0.007 

O.7I      O.OOg 

A12O3  

i8.-u       .180 

18.78          .184 

ZrOa. 

none 

o.  10 

FeaO3  

3.77         .024. 

2  .60           .OI7 

PlOc. 

0.38       .003 

O.22          .OO2 

FeO  

I  .  88       .  026 

2  86        040 

SO, 

o.o? 

none 

MgO  .. 

1  .  73         .O/12 

i  68       .  042 

MnO 

n.d. 

n.d. 

CaO  

38.1        060 

4  61         082 

BaO 

o.  17 

0.18 

Na    O 

K2O  

3-39       -°55 

8.  77         .OO4. 

3-i3       -0S° 
8  AS         ooo 

99.90 

99.61 

Hc\ 

CO,  

I.I4 
none 

0.99 

Sp.   gr... 

2.648  at  27° 

2.609  at  10° 

I.  Bagnorea]  ciminose  [leucite-trachyte].     Bagnorea,  Vulsinian  District.     H.  S.  Washington, 

analyst.     Jour.  Geol.,  V,  1897,  p.  370. 

II.  Bagnoreal  ciminose  [leucite-trachyte].  Monte  Venere,  Ciminian  District.     H.  S.  Washington, 
analyst.     Jour.  Geol.,  IV,  1896,  p.  849. 


68 


THE  ROMAN  COMAGMATIC  REGION. 


Norm  of  I. 


Norm  0}  II. 


Or  

-2 

->6} 

CO 

O4  ^ 

Ab  

.     17. 

/ 
20  V 

78 

17 

/ 
2O  > 

77 

17  } 

An.  .  .  . 

.      8. 

6"  ) 

3 

•  84    42             12 

f 

02      1 

1 

Ne  

.      6. 

g 

1                                      C 

04 

_ 

j 

Di..    . 

.      6. 

or  ) 

7 

T7? 

* 

Ol  

i  . 

J  t 
o?  S 

7 

IO 

I 

87 

9 

04 

Mt  .  .. 

41 

6 

27 

11  

i  . 

06 

14.31             I 

77  V 

5- 

31 

Hm... 

o. 

Rn 

Ap  . 

o. 

O4 

o 

O4 

o 

_ 

o 

r? 

JO 

JO 

Rest  .  .  . 

98 
i 

73 
36 

98 

I 

.29 
.27 

83-41 


14.88 


100.09 


99 -56 


Class 

Ratios. 
Sal 

I. 

5  .00 

Order  

Fem 
F 

12.51 

L 
K2O'  +  Na2O' 

A    8t 

Subrane.  .  . 

CaO' 
KaO' 

I.  71 

II. 

5.61 
13.08 

3-19 
1. 80 


These  two  analyses  are  very  closely  alike  and,  corresponding  to  this,  their 
magmatic  positions  are  almost  identical.  They  both  fall  well  within  the  limits  of 
all  the  subdivisions,  except  that  of  subrang,  in  which  they  are  both  rather  near 
the  sodipotassic  border,  almost  enough  so  to  be  considered  transitional,  in  which 
case  the  magmatic  name  would  be  monzonose-ciminose.  As  compared  with  the 
analyses  of  the  fiescolal  ciminose,  this  type  is  markedly  higher  in  alumina  and  also 
in  potash  and  soda,  a  point  which  will  be  referred  to  in  discussing  the  mode. 

Mode. — The  modes  of  the  two  specimens  analyzed  were  determined  and  may 
be  discussed  separately.  In  neither  case  could  the  colorless  constituents  of  the 
groundmass,  orthoclase,  labradorite,  and  nephelite  be  accurately  distinguished  in 
the  application  of  Rosiwal's  method,  so  that  these  are  stated  together. 


CALCULATED. 

MEASURED. 

Soda  -orthoclase,  Or2Abi  

60.7) 

«-3f 

3-5) 
8.8 

8-9  I 
i.i  ) 

4-7 

I.O 

Vol.  %. 

77.8  x 

9-4   X 
9-4   X 

0-3   X 
3-i   X 

2.6 

2-5 
2-5 

2.9 

5-2 

Sp.  gr. 

=  202.3 

=   23.5 
=    31.0 
=     0.9 

=     16.1 

Wt.  %. 

73-9 
8.6 
"•3 

°-3 
5-9 

Labradorite,  Abi  Anj  

Nephelite    

Leucite  

Autrite  .  . 

Olivine  

Biotite    

Magnetite  

Apatite  

IOO.O 

IOO.O 

273.8 

IOO.O 

PETROGRAPHY. 


69 


In  the  case  of  the  Bagnorea  specimen  the  phenocrysts,  the  leucite,  the  augite  and 
oh' vine  (measured  together),  and  the  magnetite  were  readily  measured,  the  detailed 
reductions  to  weights  being  found  in  the  formal  description.  In  calculating  the 
mode  from  the  norm,  after  estimation  of  the  augite  on  the  usual  basis  as  to  com- 
position, the  amount  of  leucite  determined  by  the  microscopic  measurements  was 
assumed  to  be  correct,  since  that  of  the  nephelite  could  not  be  thus  determined 
and  a  knowledge  of  the  amount  of  one  of  the  lenads  was  necessary. 

As  the  Monte  Venere  rock  contains  no  nephelite,  all  the  soda  was  assumed  to 
go  into  the  albite  molecule;  and  as  it  contains  some  biotite  but  no  olivine,  the 
normative  olivine  was  combined  with  some  leucite  to  form  modal  biotite.  After 
these  readjustments  and  the  calculation  of  the  augite  of  the  usual  composition,  the 
remaining  silica  was  distributed  between  orthoclase  and  leucite  by  the  regular 
equations. 


CALCULATED. 


MEASURED. 


Soda-orthoclase,  OrsAb4. 
Labradorite,  Ab,An2  . . . 

Leucite 

Augite 

Biotite 

Magnetite 

Apatite 


47-7 

16.3 

16.9 

10.3 

3-7 

3-8 

o-5 


Vol.  %.     Sp.  gr. 
68.7    X   2.6  = 

X 


20.6 

8.0 
0.7 

2.O 


2-5 

X  3-3 
X  2.9 
X  5-2 


178.6 

Si-5 
26.4 

2.O 
IO.4 


Wt.  %. 
66.5 

19.2 
IO.O 

0.8 
3-5 


268.9     100.0 


It  will  be  seen  that  the  calculated  and  measured  modes  in  both  cases  agree  very 
well  and  that  the  influence  of  overlapping  has  not  been  serious;  indeed  in  the  case 
of  the  Monte  Venere  rock  the  relations  of  light  and  dark  minerals  in  the  two  modes 
are  the  reverse  of  those  ordinarily  obtained,  the  light  and  dark  minerals  being 
respectively  less  and  more  in  the  calculated  than  in  the  measured  mode. 

As  compared  with  each  other  the  modes  of  the  two  rocks  are  closely  alike, 
though  there  are  some  differences,  as  the  presence  of  nephelite  and  olivine  in  the 
Bagnorea  rock  and  the  presence  of  biotite  in  that  from  Monte  Venere.  But  these 
differences  are  of  slight  moment,  and  may  be  regarded  as  negligible,  at  least  for  the 
present,  when  very  fine  distinctions  are  inadvisable. 

Compared  with  the  norms  the  two  modes  are  evidently  abnormative,  especially 
in  the  replacement  of  nephelite  (in  whole  or  in  part)  and  orthoclase  by  leucite,  the 
other  divergencies  being  negligible,  so  that  the  type  may  be  spoken  of  as  leucite 
alfersalphyro-monzonose-ciminose. 

It  is  of  some  interest  to  compare  the  fiescolal  ciminose  [ciminite]  with  this  type 
as  regards  chemical  composition  and  mode.  The  chemical  differences  between  the 
two,  shown  by  the  lower  alumina  and  alkalis  of  the  fiescolal  type,  have  been  adverted 
to  above,  and  it  is  quite  clear  that  the  modal  differences,  and  especially  the  presence 
of  leucite,  are  largely  dependent  on  these  chemical  differences.  Indeed,  in  the  case 


70  THE  ROMAN  COMAGMATIC  REGION. 

of  the  fiescolal  type  study  of  the  norm  and  mode  shows  clearly  that  the  presence  of 
leucite  in  the  rock  to  a  notable  extent  would  be  most  improbable  on  account  of 
the  presence  of  low  alkalis,  especially  in  view  of  considerable  amounts  of  ferrous 
oxide  and  magnesia,  which  would  allow  the  establishment  of  a  silica  equilibrium  by 
the  formation  of  olivine  and  hypersthene,  the  latter  entering  the  augite  molecule. 

Occurrence. — This  type  is  not  very  abundant,  but  probably  more  so  than  the 
number  of  specimens  which  I  collected  would  indicate.     It  seems  to  be  confined  to 
the  Vulsinian   and  Ciminian  districts,  prominent  localities  being  the  quarries  at 
Bagnorea  in  the  former  and  the  flows  of  Monte  Venere  and  one  on  the  south  shon 
of  Lake  Vico  in  the  latter. 

Name. — The  name  of  the  type  is  derived  from  that  of  the  locality  of  Bagnorea, 
near  Orvieto,  where  the  rock  is  quarried  extensively  for  paving  stones. 

In  the  prevailing  systems  the  names  commonly  bestowed  on  these  rocks  vary 
with  different  observers,  according  to  the  particular  minerals  on  which  stress  is  laid. 
Thus  the  Bagnorea  rock  was  formerly  called  by  me  leucite-phonolite  on  account  of 
the  nephelite  cement  present,  while  Bucca  calls  it  a  leucite-trachyte  and  Moderni 
a  leucitophyre.  Similarly  the  rock  of  Monte  Venere  was  called  by  me  a  leucite- 
trachyte,  while  Mercalli  and  Sabatini  consider  it  a  leucite-tephrite,  on  account  oi 
the  presence  of  labradorite,  even  in  small  amount.  As  may  be  gathered  from  the 
description  above,  and  as  is  more  evident  from  the  discussion  of  the  mode,  the 
amounts  of  both  labradorite  and  nephelite  are  so  small  that  it  would  seem  unwise 
in  bestowing  a  name,  to  allow  their  presence  to  offset  the  much  more  abundant 
orthoclase.  On  this  account,  in  the  prevailing  systems,  the  name  of  leucite-trachyte 
is  the  most  appropriate. 

BAQNOREAL  CIMINOSE.    II.  5.  2.  2. 

Megascopic  characters. — Light  gray,  compact,  porphyritic.  Leucite  phenocrysts  few, 
i  to  5  mm.,  irregular,  inconspicuous.  Augite  phenocrysts  few,  i  to  2  mm.,  black,  prismatic, 
inconspicuous.  Biotite  phenocrysts  few,  i  to  2  mm.,  tabular,  bronzy,  sometimes  wanting. 
Groundmass  light  gray,  aphanitic. 

Microscopic  characters. — Holocrystalline  to  percrystalline,  megaporphyritic,  dopatic. 
Phenocrysts:  10  to  15  per  cent,  leucite,  augite,  biotite,  orthoclase,  labradorite.  Groundmass: 
90  to  85  per  cent,  orthoclase,  labradorite,  leucite,  augite,  magnetite;  also  olivine,  biotite,  and 
nephelite,  not  always  present.  Glass  base  rare. 

Orthoclase,  from  Or2Abi  to  OrjAbf — Phenocrysts:  about  2  per  cent,  not  notable  mega- 
scopically,  0.5  to  2.0  mm.,  subhedral,  stout  prismatic,  twinned.  Groundmass:  45  to  55  per 
cent,  0.05  to  o.  2  mm.,  anhedral,  prismatic  and  equant,  arrangement  of  laths  diverse. 

Labradorite,  A^Anj. — Phenocrysts:  about  i  per  cent,  not  notable  megascopically,  0.5 
to  2.omm.,  subhedral,  stout  prismatic,  sometimes  clustered,  twinned.  Groundmass:  about 
10  to  15  per  cent,  0.03  to  o.  i  mm.,  anhedral,  prismatic,  arrangement  diverse. 

Leucite. — Phenocrysts:  about  5  to  10  per  cent,  0.5  to  5.0  mm.,  subhedral  to  anhedral, 
equant,  few  inclusions.  Groundmass:  about  5  to  10  per  cent,  o.i  to  0.5  mm.,  anhedral, 
equant,  round  sections,  few  inclusions. 

Augite. — Phenocrysts:  about  3  per  cent,  0.2  to  2.0  mm.,  subhedral  to  anhedral,  pris- 
matic and  equant,  often  fragmentary,  pale  greenish-gray  or  colorless.  Groundmass:  about  7  per 
cent,  o. 02  to  0.05  mm.,  anhedral,  prismatic  and  equant,  colorless  or  very  pale  gray. 


PETROGRAPHY.  71 

Biotite.— Phenocrysts:  about  3  per  cent,  not  always  present,  0.2  to  20  mm.,  anhedral, 
thick  tabular,  brown,  usually  much  altered. 

Olivine. — Groundmass:  about  i  per  cent,  not  always  present,  0.03  to  0.2  mm.,  anhedral, 
equant. 

Nephelite. — Groundmass:  about  3  per  cent,  not  always  present,  formless,  interstitial  cement. 

Magnetite. — Groundmass:  about  4  per  cent,  o.oi  to  0.03  mm.,  anhedral,  equant. 

Apatite. — Groundmass:    about  i  per  cent,  o.oi  to  0.04  mm.,  anhedral,  prismatic. 

Chemical  composition  and  norm  as  on  p.  67. 

Type  specimens  from  Bagnorea,  Vulsinian  District,  and  Monte  Venere,  Ciminian  District. 

II.  6-5.  2.  2.    Martinal  Vicose-Ciminose  [Leucite-Tephrite,  San  Martino  Type]. 

Megascopic  characters. — This  type  is  rather  light  gray  and  resembles  the  bagn- 
oreal  ciminose.  It  is,  however,  much  more  highly  porphyritic,  phenocrysts  of 
leucite  and  augite  being  abundant,  though  somewhat  smaller  than  in  the  preceding 
type.  Those  of  leucite  are  seldom  over  i  mm.  in  diameter  and  are  very  incon- 
spicuous, while  those  of  augite,  in  black  prisms  from  i  to  2  mm.  long,  are  much 
more  so.  The  groundmass  is  a  rather  dark  gray,  dense,  and  aphanitic. 

Microscopic  characters. — In  thin  section  the  leucite  phenocrysts  are  seen  to  be 
either  subhedral  or  sometimes  quite  anhedral,  from  0.5  to  i  mm.,  in  diameter.  They 
carry  few  inclusions,  which  consist  of  minute  dusty  grains,  generally  gathered  toward 
the  center.  The  subhedral  augite  prismoids  are  somewhat  more  abundant,  are 
much  cracked  and  fragmentary,  though  mostly  with  crystal  planes  remaining,  and 
are  either  colorless  or  of  an  almost  imperceptible  greenish  tinge.  They  contain  no 
inclusions.  These  two  minerals  constitute  the  only  phenocrysts,  none  of  feldspar 
or  biotite  having  been  observed.  The  leucite  phenocrysts  commonly  occur  in 
clusters  of  several  individuals. 

The  groundmass  is  somewhat  indefinite  and  difficult  of  exact  study.  There 
are  numerous  small,  colorless  augite  anhedra,  which  are  either  prismatic,  equant, 
or  irregular,  and  some  small  grains  of  magnetite,  but  none  of  the  interstitial  biotite 
seen  in  the  previous  type.  These  minerals  are  embedded  in  a  colorless  mass, 
which  the  crossed  nicols  resolve  into  very  small,  round  anhedra  of  leucite,  present 
in  large  numbers,  indefinitely  outlined  prismoids  of  feldspar,  which  frequently  show 
multiple  twinning,  though  the  grays  are  light  and  the  structure  is  best  seen  with  the 
aid  of  the  selenite  plate.  Along  with  these  are  small  flakes  and  indefinite  interstitial 
areas  of  a  colorless,  feebly  birefringent  substance,  which,  however,  does  not  seem 
to  be  nephelite,  judging  from  the  colors  shown  with  the  selenite  plate,  but  is  rather 
to  be  referred  to  one  of  the  feldspars,  probably  orthoclase.  No  glass  could  be  iden- 
tified with  certainty. 

Chemical  composition. — One  analysis,  hitherto  unpublished,  was  made  of  this 
type.  In  its  lower  silica  and  alumina,  as  well  as  in  its  somewhat  higher  bivalent 
oxides,  this  analysis  differs  very  considerably  from  those  of  the  other  ones  of  ciminose 
rocks.  The  explanation  lies  in  the  fact  that,  while  this  rock  is  well  within  the 
borders  of  the  class,  rang,  and  subrang,  it  lies  close  to  the  border  of  the  lendofelic 
order.  So  close,  indeed,  is  it  that  the  type  may  be  regarded  as  transitional 


72 


THE  ROMAN  COMAGMATIC  REGION. 


between  the  subrangs  ciminose  (II.  5.  2.  2),  where  it  falls,  and  that  of  vicose  (II. 
6.  2.  2),  so  that  it  should  be  called  strictly  a  vicose-ciminose. 

Chemical  Composition  of  Martinal  Vicose-ciminose  \Leucite-lephrite]. 


CONSTITUENT. 

I. 

CONSTITUENT. 

I. 

SiO2 

C2  .  Id. 

0.869 

H2O  —          

O.  OQ 

A12O3  

1$  .01 

.iq6 

CO2  

none 

Fe,O,. 

2.  1C 

.  OIJ. 

TiOj  

1.22       O 

OIS 

FeO..               

e  .  TO 

.072 

ZrO2  

trace 

MgO  

e.  II 

.128 

P,Oc   .  . 

o.  24 

002 

CaO  

8.  ere 

.  1^3 

SO,  .  . 

0.07 

OOI 

Na2O  

I  ,O2 

.o^i 

MnO  

n.d 

K2O  

7  '.  24 

.077 

BaO  

o.  20 

H,o+ 

100.13 

Martinal  vicose-ciminose  [leucite-tephrite].     Fosso  della  Parchetta,  San  Martino,  Ciminian  Dis 
trict.     H.  S.  Washington,  analyst. 


Or  

Norm. 
.42.81  1 

Ab  

.     2.  IO  V 

An  

C 
.  .  17.  34    1 

j 

Ne  

.  7.67 

7.67  ' 

Di  

/•"/ 

.  .21  .07  / 

Ol  

.    «?.6q  \ 

27.66 

Mt  

n.  

2  28  \ 

5-53 

Ap.. 

.   0.64 

0.64 

Rest  

99-75 
0.46 

Class 


65.92 


33-83 


Order. 


Ratios. 

Sal 

'  'Fern 

F 
'"L 


Rang 


Subrang. 


K2O'+Na2O' 
CaO' 

K2O' 
'Na2O' 


=  i-9S 


:7-59 


=  2.48 


100.25 


Mode.  —  While  the  amounts  of  the  phenocrysts  of  leucite  and  augite  could  be 
accurately  determined  under  the  microscope,  the  constituents  of  the  groundmass 
did  not  lend  themselves  to  determination  by  Rosiwal's  method  on  account  of  the 
indefinite  character  of  the  salic  components,  which  had  to  be  reckoned  together. 
The  following  were  the  results  obtained: 

Vol.  %.     Sp.  gr. 
Feldspars,  etc  .............  47  .  5   X  2.6 

Leucite  phenocrysts  .......   11.2   X  2.5   • 

Augite  phenocrysts  ........    12.2    X  3.3   = 

Augite  in  groundmass  ......   15-6  X  3.3 

Magnetite  ................     3.5   X 


5-2 


149-5 
28.0 


5°-4 
18.2 


Wt.  %. 
52.0 

9-7 
14  o 
17.9 

6-3 


287.4      99.9 


In  calculating  the  mode  from  the  norm  it  was  assumed  that  no  nephelite  was 
present,  and  that  the  alkali-feldspar  had  the  composition  OrjAbj.  The  assump- 
tion was  made  in  regard  to  this  rather  than  to  the  soda-lima  feldspar,  since  it  was 
difficult,  if  not  impossible,  to  determine  the  average  composition  of  this  latter  opti- 
cally. The  calculated  composition  of  Ab4An3,  an  andesine,  agrees  as  well  as  could 
be  expected  with  the  appearance  under  the  microscope.  The  calculated  mode  is 
as  follows: 


PETROGRAPHY.  73 

Orthoclase,  Or,  Abt 5.4 

Andesine,  Ab4An3 23.2 

Leucite „ 31.5 

Augite.. 33.8 

Magnetite 5.5 

Apatite 0.6 


From  the  above  it  will  be  seen  that  about  21  per  cent  of  leucite  is  in  the  ground- 
mass,  the  rest  of  the  salic  portion  of  this  being  of  feldspars,  altogether  amounting 
to  about  31  per  cent.  The  calculated  and  measured  amounts  of  augite  and  mag- 
netite agree  well,  though  the  measured  amount  of  the  former  is  slightly  lower  than 
the  calculated,  the  reverse  being  true  of  the  ores.  As  compared  with  the  norm, 
the  only  notable  difference  lies  in  the  modal  presence  of  leucite,  replacing  part  of 
the  orthoclase,  and  the  total  absence  of  nephelite.  The  type  may  then  be  called  a 
leucite-alfersalphyro-ciminose. 

Occurrence. — This  type,  so  far  as  known,  is  found  only  in  the  neighborhood  of 
San  Martino,  on  the  northwestern  slope  of  the  Vico  Volcano  in  the  Ciminian  District, 
the  best  locality  being  at  a  quarry  near  the  bridge  over  the  Fosso  della  Parchetta. 
Rocks  of  the  same  type  were  also  obtained  behind  the  church  of  San  Martino,  above 
the  town,  and  as  blocks  at  the  Ponte  di  Cetti,  on  the  Via  Aurelia,  about  7  km.  south- 
west of  Viterbo,  these  last  being  somewhat  decomposed. 

Name. — The  type  name  is  derived  from  the  locality  where  the  type  occurs. 

In  the  prevailing  systems  of  classification  this  type  would  be  regarded  as  a 
leucite-tephrite,  on  account  of  the  very  considerable  amount  of  plagioclase  and 
the  small  quantity  of  orthoclase. 

MARTINAL  VICOSE-CIMINOSE.    II.  6-5.  2.  2. 

Megascopic  characters. — Light  gray,  compact,  highly  porphyritic.  Leucite  phenocryst? 
abundant,  but  very  small  and  inconspicuous.  Augite  phenocrysts  abundant,  small,  prismatic, 
black.  Groundmass  light  gray  and  aphanitic. 

Microscopic  characters. — Holocrystalline,  mediophyric,  dopatic.  Phenocrysts:  about  25  per 
cent,  leucite,  augite.  Groundmass:  about  75  per  cent,  somewhat  felted  fabric,  andesine,  leucite, 
augite,  orthoclase,  magnetite,  apatite. 

Orthoclase,  O^Ab,. — Phenocrysts  none.  Groundmass:  about  5  per  cent,  0.05  to  o.io  mm., 
anhedral,  prismatic  and  equant,  also  interstitial  between  the  other  constituents. 

Andesine,  Ab4An3. — Phenocrysts  none.  Groundmass:  about  23  per  cent,  0.05  to  o.io 
mm.,  anhedral,  prismatic,  outlines  rather  indefinite,  twinned,  arrangement  diverse. 

Leucite. — Phenocrysts:  about  n  per  cent,  0.4  to  i.omm.,  subhedral,  equant,  often  clus- 
tered, few  dusty  inclusions.  Groundmass:  about  20  per  cent,  0.02  to  0.05  mm.,  anhedral, 
equant,  free  from  inclusions,  often  difficult  to  distinguish  from  the  groundmass  orthoclase. 

Augite. — Phenocrysts:  about  12  per  cent,  0.5  to  2.0  mm.,  subhedral,  prismatic,  fusiform 
and  fragmentary,  colorless,  no  inclusions.  Groundmass:  about  20  per  cent,  0.02  to  0.05  mm., 
anhedral,  prismatic,  equant  or  irregular,  colorless. 

Magnetite. — Groundmass:    about  4  per  cent,  0.02  to  0.04  mm.,  anhedral,  equant. 

Chemical  composition  and  norm  as  on  p.  72. 

Type  specimen  from  Fosso  della  Parchetta,  San  Martino,  Vico  Volcano,  Ciminian  District. 


74  THE  ROMAN  COMAGMATIC  REGION. 

II.  5.  2.  3.    Arsal  Monzonose  [Vulsinite,  Arso  Type]. 

Megascopic  characters. — Rocks  of  this  type  are  very  similar  to  those  belonging 
to  arsal  vulsinose  and  ciminose  described  above,  both  megascopically  and  micro- 
scopically; so  much  so,  indeed,  that  they  are  quite  indistinguishable  in  the  hand 
specimen. 

The  color  is  a  pure  gray  which  may  vary  from  rather  light  to  rather  dark.  Glis- 
tening, white,  prismatic  phenocrysts  of  feldspar,  from  5  to  20  mm.  long,  are  rather 
abundant  and  are  more  or  less  conspicuous  according  as  the  groundmass  is  dark 
or  light.  With  these  are  seen,  but  in  very  small  amount,  small  black  prisms  of 
augite,  and  in  still  less  quantity  either  small  tables  of  biotite  or  grains  of  yellow 
olivine.  The  groundmass  is  dense  and  aphanitic.  Most  of  the  rocks  of  this  type 
in  the  Roman  Region  are  compact,  but  occasionally  more  or  less  vesicular  forms  are 
found. 

Microscopic  characters. — The  feldspar  phenocrysts  are  seen  in  thin  section  to 
be  of  both  alkali  and  soda-lime  feldspar,  the  amount  of  the  former  surpassing  that 
of  the  latter.  While  the  orthoclase  is  more  or  less  sodic  it  does  not  show  any  of 
the  microperthitic  or  other  micro-structures  which  are  so  common  in  soda-ortho- 
clase.  The  stout  prisms  are  euhedral  or  subhedral,  often  twinned  according  to  the 
Carlsbad  law,  and  carry  few  inclusions.  Those  of  soda-lime  feldspar  are  similar  in 
form  and  show  the  usual  multiple  twinning.  The  extinction  angles  vary  somewhat, 
but  in  most  cases  indicate  that  the  composition  is  near  the  border  of  andesine  and 
labradorite,  varying  from  Ab4An3  in  the  Arso  rock  to  Ab3An4  in  that  from  Poggio 
Cavaliere,  and  even  slightly  more  calcic.  It  will  be  recalled  that  in  the  homologous 
types  of  vulsinose  and  ciminose  the  plagioclase  was  a  labradorite  about  AbIAn2,  or 
even  an  almost  pure  anorthite,  the  composition  being,  of  course,  correlated  with 
the  dopotassic  or  sodipotassic  character  of  the  subrang. 

The  augite  phenocrysts  call  for  little  remark.  They  vary  in  length  from  i  to  i 
5  mm.,  are  usually  anhedral,  in  stout  prismoids  or  fragments  of  these,  and  are  of 
the  common  very  pale-gray  color,  sometimes  slightly  greenish-yellow.  The  rare 
biotite  phenocrysts,  which  are  not  present  in  all  specimens,  are  in  thick  tables,  i  to 
3  mm.  wide,  of  the  common  brown  color  and  usually  much  altered.  In  some 
specimens,  as  that  from  L'Arso,  these  are  replaced  by  phenocrysts  of  olivine,  from 
i  to  2  mm.  in  diameter,  mostly  anhedral,  occasionally  with  crystal  planes,  colorless 
in  thin  section,  and  quite  fresh. 

The  groundmass  is  composed  very  largely  of  slender  laths  of  feldspar,  those  of 
orthoclase  preponderating  over  those  of  soda-lime  feldspar,  which  is  an  andesine- 
labradorite.  In  the  Arso  rock  these  small  laths  are  branched  at  the  ends  and  often 
slightly  curved.  Their  arrangement  is  subparallel,  giving  rise  to  a  trachytic  fabric. 
With  them  are  seen  rarely,  as  in  the  Arso  specimens,  very  small,  round  anhedra  of 
leucite,  but  the  amount  of  this  is  quite  negligible,  not  over  2  per  cent,  and  it  is  note- 
worthy that  it  does  not  occur  in  those  rocks  which  show  ^biotite  phenocrysts,  as  that 
from  Poggio  Cavaliere.  The  presence  of  leucite  in  the  Arso  rock  was  first  noted  by 


PETROGRAPHY. 


75 


vom  Rath,  but  it  was  not  observed  by  Fuchs.  Its  identity  was  established  by  the 
close  resemblance  to  other  occurrences,  the  evidences  of  the  birefringence  and 
twinning,  and  the  presence  in  many  cases  of  the  characteristic  minute  inclusions  of 
augite  which  are  centrally  arranged.  The  groundmass  also  carries  the  usual  small 
colorless  augite  anhedra,  in  prisms  and  grains,  and  some  small  grains  of  magnetite. 
The  specimens  typically  are  holocrystalline,  those  from  Cavaliere  showing  a  little 
nephelite  cement,  but  those  from  L'Arso  carry  a  little  glass,  either  colorless  or  brown- 
ish and  often  dusty  with  minute  indeterminable  microlites. 

Chemical  composition. — Analyses  of  two  specimens  of  this  type  are  found  below, 
one  of  which  has  already  been  published  in  incomplete  form,  together  with  one  by 
Riva  of  another  type  of  monzonose  which  may  be  called  bolsenal  from  the  descrip- 
tion by  De  Lorenzo,  as  well  as  an  early  one  by  Fuchs  of  the  Arso  rock,  and  also  one 
of  a  monzonose  from  Montana. 


Chemical  Composition  of  Arsal  Monzonose  [Vulsinite]. 


I. 

II. 

III. 

IV. 

V. 

SiO2  

55.22     0.920 

56.75     0.946 

54.72     0.912 

57.73 

55.23 

A12O3  

18.43       .181 

18.03       .177 

10.60       .192 

17.85 

I8.7I 

Fe2O3  

2  .  O2           -°I3 

2.22           .OI4 

2.4?        .01  "5 

4.44 

4.00 

FeO   

•?.  10       .  04? 

3  .  04       .  042 

3.00       .  04? 

3.QO 

2.06 

MgO  .  . 

2.  75         .060 

2.  O2          .051 

i  .  oo       .  048 

I.  77 

1.  85 

CaO  

C  .  7O          .  IO2 

4.68          .084 

<  .  oo       .  080 

3.6<C 

3.62 

Na2O  

2.  ?o       .  0^6 

4.8?           .078 

3.C2       .o<;6 

•7.  77 

4.02 

K3O  

7.<;8       .081 

C.O2           .063 

6.87       .073 

7.6"? 

6-43 

H2O  +   .    .. 

o.  4.6  ) 

1.84 

H2O—  

u  4    ( 

O.  20   ) 

0.18 

2.08 

0.09 

CO2  

none 

none 

TiO2  

o  87       .on 

1.24       .016 

o  .  65       .  008 

0.42 

ZrO3  

o.  10 

PjOc    .   . 

0.21       .002 

o  .  34       .  002 

trace 

trace 

0.58 

SO,  .. 

O.  II 

0.23 

Cl  

O.  II 

o.oc. 

n.d. 

0.32 

CraO3  

none 

MnO  

nd 

n.d. 

trace 

BaO  

0.07 

0.46 

100.41 

99-38 

99-93 

100.85 

100.27 

I.  Arsal  monzonose  [vulsinite].      Poggio  Cavaliere,  Lake  Vico,  Ciminian  District      Wash- 
ington, analyst. 

II.  Arsal  monzonose  [trachyte,  ciminite].    L'Arso,  Ischia,  Campanian  District.     Washington, 
analyst.     Am.  Jour.  Sci.,  VIII,  1899,  p.  290. 

III.  Bolsenal    monzonose    ["  trachydolerite, "    vulsinite].       Ejected    block,  Astroni    Volcano, 

Phlegrean  Fields,   Campanian  District.     Riva,  analyst.     De  Lorenzo  and  Riva, 
Alt.  Ace.  Sc.  Nap.,  XI,  1902,  p.  37. 

IV.  Arsal  monzonose  [trachyte].    L'Arso,  Ischia,  Campanian  District.    Fuchs,  analyst.     Min. 

Pet.  Mitth.,  1872,  p.  230. 

V.  Aspenal  monzonose  [gauteite].      Aspen  Creek,  Highwood  Mountains,  Montana.      Foote, 
analyst.     L.  V.  Pirsson,  Bull.  U.  S.  G.  S.  No.  237,  1905,  p.  134. 


76 


THE  ROMAN  COMAGMATIC  REGION. 


Or  .      ... 

Norms. 
I.                                         II. 

.    4?  .04  "i                                              3C.O3  ^ 

Ab       

3           /                  \                           ""       •*  /                  \ 

...     14.  Q3  }•  72.  2O  )                            32.  4Q  5-77.  S3) 

An        

12.23)             [•  80.01         10.01)             >82.O7 

Ne 

7.8l         7.8l)                              4.  S4         4.C4J 

Di  

II  .37  )                                                 9.  12 

Ol  

2.85  \  I4'22                                  1.67       I0'79 

Mt  

3.O2)            ..           IO.42                3.2?                to       17.28 

11  

1.67      4-69                       I    I      5-68       7 

Ap.  . 

0.51       o-$i                        0.81       0.81 

Rest  

99-43                                    99-35 
i  .03                                      o.  20 

100.46                                  99-64 

Ratios. 

I.               II. 
Sal 

"Fern 
Order..            ..I                         =9'24         I7'°8 

J_j 

Subraner  .  .        .  .-  —  -  —  :                =I.A*           0.8^ 

ni. 

79-58' 


4°-59 
21.48 

17-51 
4.26     4.26 
5.88^ 
3-32  S 
3-48 

1.22 


9-2O 
4.70 


83.84 


13.90 


97-74 
2-13 

99.87 


III. 

6.03 

18.68 

2.05 
1.30 


These  analyses  do  not  call  for  special  comment.  They  are  much  alike  in  all 
respects  and  the  rocks  evidently  fall  well  within  the  borders  of  the  various  divisions, 
except  that  that  from  Poggio  Cavaliere  lies  somewhat  close  to  the  border  of  the 
dopotassic  subrang,  though  not  close  enough  to  be  considered  transitional.  The 
older  analysis  of  the  Arso  rock  by  Fuchs,  and  a  still  earlier  one  by  Abich, 
resemble  that  in  II,  except  in  their  somewhat  higher  silica  and  potash  and  lower 
soda,  but  their  dates  and  the  conditions  obtaining  in  analytical  work  and  methods 
at  the  time  render  them  not  altogether  reliable. 

Mode. — The  modes  of  the  two  specimens  analyzed  by  me  were  determined  and 
are  here  described.  That  of  the  bolsenal  type  can  only  be  calculated  from  the 
norm,  as  the  description  of  De  Lorenzo  is  not  quantitative. 

In  the  case  of  Poggio  Cavaliere  rock  determination  of  the  mode  by  optical 
methods  was  rendered  difficult  by  the  size  and  arrangement  of  the  groundmass 
feldspars  and  by  the  presence  of  the  nephelite  cement.  Nor  could  the  relative  amounts 
of  the  phenocrysts  of  orthoclase  and  plagioclase  be  exactly  estimated  in  the  absence 
of  sufficiently  large  sections.  On  the  other  hand,  the  amount  of  augite,  both  in 
phenocrysts  and  in  the  groundmass,  and  of  the  biotite  phenocrysts,  could  be  closely 
judged,  the  augite  being  about  16  per  cent  and  the  biotite  about  1.8  of  the  rock  by 
weight. 

In  calculating  the  mode  from  the  norm  the  amount  of  biotite  as  thus  deter- 
mined was  assumed  to  be  correct,  and  the  proper  amounts  of  olivine  and  leucite 
molecules  were  assigned  to  it.  The  small  amount  of  olivine  remaining,  changed  to 


PETROGRAPHY.  77 

hypersthene,  was  assigned  to  the  augite  molecule,  and  the  silica  left  after  these  read- 
justments and  the  calculation  of  orthoclase  and  anorthite  was  divided  between  albite 
and  nephelite  molecules  in  the  usual  way.  The  soda-lime  feldspar  was  assumed  to 
have  the  composition  Ab3An4,  thus  leaving  a  little  soda  for  the  alkali-feldspar.  The 
results  of  this  calculation  are: 

Soda-orthoclase,  OrsAb! 52.1 

Labradorite,  Ab3  An4 17.8 

Nephelite 7.8 

Augite 17.7 

Biotite   1.7 

Magnetite 2.4 

Apatite 0.5 


These  figures  agree  with  the  appearance  of  the  section.  Thus  the  measured 
lount  of  the  two  feldspars  and  the  nephelite  base  was  about  77  per  cent,  while  the 
calculated  total  for  these  is  77.7.  The  calculated  amount  of  augite  is  slightly 
greater  than  the  measured,  and  it  is  possible  that  some  of  the  very  small  augites  of 
the  groundmass  have  been  overlooked,  but  the  divergence  is  not  large.  From  treat- 
ment of  the  section  with  acid  and  fuchsine  it  would  seem  that  the  calculated  amount 
of  nephelite  corresponds  closely  to  the  actual. 

Taking  up  the  Arso  rock  the  same  difficulites  meet  us,  as  regards  the  feldspars 
and  glass  cement,  when  it  is  attempted  to  estimate  the  mode  by  Rosiwal's  method. 
But  here,  likewise,  the  amounts  of  leucite.  augite,  and  olivine  could  be  measured 
with  a  reasonable  degree  of  certainty.  The  results  are,  for  leucite  1.6  per  cent,  for 
augite  ii  per  cent,  and  for  olivine  2.1  per  cent,  the  total  amount  of  phenocrystic  and 
groundmass  feldspars  and  glass  being  84,  and  that  of  ores  about  1.5  per  cent. 

Although  the  texture  is  somewhat  hyaline  and  the  mode  therefore  is  indeter- 
minable, strictly  speaking,  the  amount  of  glass  is  so  small  that  the  calculation  of  the 
mode  from  the  norm  may  be  attempted.  For  this  purpose  it  was  assumed  that 
the  amount  of  leucite  was  that  shown  optically  and  that  the  plagioclase  had  the 
composition  shown  below.  The  result  is: 


Soda-orthoclase,  Or4Ab3 55 . 2 

Andesine,  Ab4An3 20.1 

Leucite 1.7 

Nephelite 4.0 

Augite 13 .  i 


Olivine 1.7 

Magnetite 3.4 

Apatite 0.8 


From  the  somewhat  greater  amounts  of  augite  and  magnetite  shown  by  the 
calculated  mode,  and  the  presence  of  nephelite,  it  is  clear  that  the  glass  cement 
is  essentially  a  nephelite  in  composition  (which  is  confirmed  by  the  fact  that  it  gel- 
atinizes readily  with  acids),  and  that  it  carries  considerable  augite  and  magnetite, 
either  as  the  fine,  dusty  microlites,  or  uncrystallized  as  the  brownish  coloring 
material.  The  olivine  shown  in  the  mode  closely  approximates  to  that  of  the  norm. 

It  is  of  interest  to  note  here  that,  instead  of  the  small  amount  of  modal  biotite 


78  THE  ROMAN  COMAGMATIC  REGION. 

shown  by  the  Cavaliere  rock,  this  carries  the  leucite  and  olivine  molecules  crystal- 
lized as  these  minerals,  and  not  combined  into  biotite. 

It  is  clear  that  the  modes  of  both  specimens  are  essentially  the  same,  as  the 
amounts  of  biotite  in  the  one  and  of  leucite  and  olivine  in  the  other  are  so  small  as 
to  be  quite  negligible.  As  compared  with  the  norm  the  differences  are  negligible 
also  in  both  cases,  and  the  type  may  be  described,  neglecting  the  small  quantities  of 
alferric  phenocrysts,  as  normative  salphyro-monzonose. 

Occurrence. — This  type  is  not  abundant  in  the  Roman  Region,  occurring  rather 
sparingly  in  but  two  districts,  the  Ciminian  and  the  Campanian.  In  the  former  it 
is  best  represented  by  a  massive  flow  below  the  Poggio  Cavaliere,  on  the  south  shore 
of  Lake  Vico,  and  it  also  forms  blocks  in  the  tuffs  on  the  northwest  slope  of  the  Vico 
Volcano,  especially  near  the  Villa  Balestra  on  the  Via  Aurelia,  about  4  km.  south- 
west of  Viterbo.  In  the  Campanian  District  it  forms  the  flow  of  1302  A.  D.,  called 
L'Arso,  on  the  island  of  Ischia,  and  a  vesicular  form  occurs  at  Le  Cremate,  the 
point  of  eruption  above  the  flow. 

The  type  of  bolsenal  monzonose,  which  is  based  provisionally  on  the  descrip- 
tion and  analysis  of  De  Lorenzo  and  Riva,  is  met  with  as  blocks  at  the  Astroni  Vol- 
cano in  the  Phlegrean  Fields,  but  is  not  yet  known  outside  the  Campanian  District, 
though  it  probably  occurs  elsewhere. 

Name. — The  subrang  name  is  derived  from  that  of  Brogger's  group  of  mon- 
zonites,  many  typical  examples  of  which  fall  in  this  subrang.  The  type  name  is 
derived  from  the  well-known  locality  of  L'Arso.  The  possibility  and  the  advisabil- 
ity of  distinguishing  two  types,  the  arsal  and  the  vetrallal,  based  on  the  conspicuous- 
ness  of  the  feldspar  phenocrysts  and  the  presence  of  olivine  or  biotite,  has  already 
been  discussed  under  the  type  of  arsal  vulsinose-ciminose  (p.  61). 

The  remarks  on  the  position  of  the  homologous  types  of  vulsinose  and  ciminose 
in  the  prevailing  systems  are  applicable  here.  In  general  these  rocks  would  be  called 
trachytes,  but  they  are  commonly  recognized  as  rather  abnormal.  The  Poggio 
Cavaliere  rock  would  be  a  typical  vulsinite,  while  that  of  L'Arso  is  distinguished  by 
Rosenbusch  from  the  true  trachytes  as  the  type  of  the  Arso-typus,  is  often  spoken 
of  as  an  olivine-trachyte,  and  was  formerly  called  a  ciminite  by  me.  It  is  note- 
worthy that  the  present  measurements  show  the  very  small  amount  of  olivine  actually 
present,  and  emphasize  again  the  prevalent  tendency  to  exaggerate  the  importance 
of  unusual  constituents  which  may  be  present  in  negligible  amounts. 

De  Lorenzo  and  Riva  call  attention  to  the  chemical  divergence  of  the  bolsenal 
monzonose  from  the  similar  types  which  fall  in  vulsinose,  and  make  this  clear  by 
naming  the  vulsinose  rocks  vulsinite,  while  the  monzonose  type  is  referred  to  as  a 
trachydolerite. 

ARSAL  MONZONOSE.    II.  5.  2.  3. 

Megascopic  characters. — Medium  gray,  usually  compact,  porphyritic.  Feldspar  pheno- 
crysts common,  3  to  10  mm.  long,  stout  prismatic,  white,  rather  conspicuous.  Augite  pheno- 
crysts rare,  small,  prismatic,  black.  Biotite  phenocrysts  not  always  present,  few,  small,  tabular, 


PETROGRAPHY. 


79 


black.  Olivine  phenocrysts  not  always  present,  few,  small,  yellow.  Groundmass:  gray, 
aphanitic. 

Microscopic  characters. — Holocrystalline  to  percrystalline,  magnophyric,  dopatic.  Pheno- 
crysts: about  20  per  cent,  orthoclase,  andesine-labradorite,  augite,  biotite  or  olivine  sometimes. 
Groundmass:  about  80  per  cent,  either  holocrystalline  or  with  a  very  little  glass,  trachytic 
fabric,  orthoclase,  andesine-labradorite,  augite,  leucite  (sometimes),  nephelite  (sometimes), 
magnetite,  apatite,  glass  (sometimes). 

Orthoclase,  Or5Abj  to  Or4Ab3. — Phenocrysts:  about  10  per  cent,  i  to  20  mm.,  euhedral 
to  subhedral,  stout  prismatic,  Carlsbad  twinning,  inclusions  rare.  Groundmass:  about  40  per 
cent,  0.05  to  o.io  mm.,  anhedral,  prismatic,  sometimes  branched,  with  diverse  to  subparallel 
arrangement. 

Andesine-labradorite,  Ab3An4  to  Ab4An3. — Phenocrysts:  about  5  per  cent,  5  to  2omm.i 
subhedral,  stout  prismatic,  usually  twinned,  inclusions  rare.  Groundmass:  about  15  per  cent, 
0.05  to  o.  10  mm.,  anhedral,  prismatic,  arrangement  diverse  to  subparallel. 

Leucite. — Phenocrysts  none.  Groundmass;  not  always  present,  sometimes  about  2  per 
cent,  0.05  to  o.io  mm.,  anhedral  round  equant,  faintly  birefringent,  some  inclusions  of  augite 
microlites  centrally  arranged. 

Nephelite. — Groundmass:  not  always  present,  up  to  about  8  per  cent,  anhedral,  as  cement 
interstitial  between  the  feldspars. 

Augite. — Phenocrysts:  about  3  per  cent,  0.5  to  2.0  mm.,  subhedral  to  anhedral,  stout 
prismatic  and  fragmentary,  gray  or  very  pale  greenish-yellow,  few  inclusions.  Groundmass: 
about  10  per  cent,  0.02  to  0.20  mm.,  anhedral,  prismatic,  colorless  or  very  pale  yellowish. 

Biotite. — Phenocrysts:  not  always  present,  sometimes  about  2  per  cent,  i  to  3  mm.,  anhe- 
dral, thick  tabular,  much  altered  to  granular  aggregate. 

Olivine. — Phenocrysts:  not  always  present,  sometimes  about  2  per  cent,  i  to  2  mm.,  sub- 
hedral to  anhedral,  fusiform  to  equant,  colorless  and  usually  fresh. 

Magnetite. — Groundmass:  about  3  per  cent,  o.oi  to  0.03  mm.,  anhedral,  equant. 

Apatite. — Groundmass:   about  i  per  cent,  0.02  to  0.05  mm.,  subhedral,  prismatic. 

Glass. — Groundmass:  not  always  present,  about  5  per  cent,  usually  colorless,  sometimes 
brown,  dusty  with  minute  microlites. 

Chemical  composition  and  norm  as  on  p.  75 

Type  specimens  from  Poggio  Cavaliere,  south  shore  of  Lake  Vico,  Ciminian  District,  and 
from  L'Arso,  Ischia,  Campanian  District. 

II.  5.  2-3.  2.     Foglianal  Ciminose-Auruncose  [Leucite-Tephrite,  Viterbo  Type]. 

Megascopic  characters. — This  type  much  resembles  in  the  hand  specimen  the 
viterbal  vulsinose  and  ciminose  already  described,  though  the  only  known  example 
differs  texturally  in  being  somewhat  smaller  in  grain,  both  as  to  phenocrysts  and 
groundmass.  The  rock  is  a  leucite  melaphyre,  leucite  phenocrysts,  which  vary  in 
diameter  from  i  to  5  mm.  and  which  are  usually  euhedral,  constituting  about  one- 
fifth  of  the  rock  volume.  There  are  much  less  abundant  phenocrysts  of  feldspar 
and  augite,  but  on  account  of  their  small  dimensions  these  are  very  inconspicuous. 
The  groundmass  is  rather  dark  gray  and  aphanitic.  The  only  specimen  obtained 
is  somewhat  vesicular. 

Microscopic  characters. — In  thin  section  the  leucite  phenocrysts  present  no 
remarkable  features.  They  are  euhedral,  sometimes  clustered,  with  very  faint  double 
refraction,  and  carry  few  inclusions,  mostly  small  round  dots  of  glass  and  usually 


8o 


THE  ROMAN  COMAGMATIC  REGION. 


zonally  arranged  in  thin  bands  parallel  to  the  crystal  outline.  The  great  majority 
of  the  feldspar  phenocrysts  are  of  a  labradorite,  about  AbxAn2,  in  euhedral,  stout 
prisms,  always  twinned  according  to  the  usual  laws.  Phenocrysts  of  alkali -feldspar 
are  less  numerous,  are  somewhat  less  apt  to  be  euhedral,  and  show  Carlsbad  twin- 
ning. The  phenocrysts  of  augite  of  the  usual  pale-gray  color  are  very  rare,  euhedral 
to  subhedral,  but  often  in  fragments. 

The  groundmass  in  which  these  lie  is  extremely  fine-grained.  It  is  composed 
in  large  part  of  minute  feldspar  laths,  which  are  of  both  orthoclase  and  soda-lime 
feldspar,  though  their  small  dimensions  and  absence  of  twinning  render  exact 
determination  difficult.  With  them  are  many  small  prisms  and  grains  of  colorless 
augite,  small  anhedra  of  olivine,  and  some  magnetites.  Between  these  lies  a  color- 
less cement,  which  is  in  part  doubly  refracting,  and  may  be  considered  to  be  feldspar, 
since  nephelite  is  absent,  judging  from  tests  on  the  rock  powder.  In  places  this 
interstitial  substance  is  isotropic,  these  areas  being  usually  round  and  probably  of 
leucite,  though  their  very  small  size  renders  identification  difficult.  Glass  may  also 
be  present  to  a  small  extent,  but  could  not  be  definitely  established. 

Chemical  composition. — But  one  analysis  was  made  of  this  type,  hitherto 
unpublished: 


Chemical  Composition  of  Foglianal  Ciminose-auruncose  [Leucite-tephrite]. 


I. 

I. 

SiO2  

S2.37       O 

873 

H2O—  

o.  t;i 

A12O3  

20.89 

2OJ. 

CO2  

none 

Fe2O3  

1  .  21 

008 

TiO2  

i  is 

O    OT4 

FeO  

4.44 

062 

ZrO2  

o.  07 

MgO  

I  •  74 

O44 

P2Oc    .  . 

O    "Cl 

.  OO4. 

CaO  

t;.o8 

OQI 

SO3  

none 

Na2O  

2  .QO 

O4  7 

MnO  

n.d 

K2O  

7.4.7 

080 

BaO  

o.  16 

H2O  + 

99.96 

Foglianal  ciminose-auruncose  [leucite  -tephrite].     Croce  di  San  Martino,  Monte  Vico,  Ciminian 
District.     H.  S.  Washington,  analyst. 


Or 44-48 

Ab 13.62  {•  79.; 


An  

\ 

.  .  21  .  41    ) 

•  8c.4S 

Ne  

C.04 

i  J 

Ol  

.   7  16 

7.l6  ' 

i 

Mt  

i  .  86 

3.  00 

•  1  2  .  40 

11  

.    2.13 

1  .  34 

Ap  

.     I.  34 

97-94 
H2O,  etc 2.20 

100.14 


Class 

Sal 

Order  

Fern 
F 

"L 
K2O'+Na2O 

Subrang.  . 

CaO' 
K2O' 

6.84 


'I3-38 


1-65 


1.70 


PETROGRAPHY. 


81 


This  analysis  is  not  specially  noteworthy  in  itself.  Although  the  magnesia 
may  appear  to  be  low  for  a  rock  which  carries  olivine,  the  norm  shows  that  con- 
siderable of  this  mineral  may  be  present,  more,  indeed,  than  exists  modally.  The 
norm  is  remarkable  among  the  rocks  of  the  region  ia  showing  no  diopside,  all  the 
femic  lime  being  taken  up  by  apatite.  The  alkalicalcic  subrang  is  in  harmony 
with  the  predominantly  calcic  character  of  the  feldspar. 

As  regards  its  position  in  the  quantitative  classification  the  rock  is  an  uncom- 
monly transitional  one.  It  falls  well  within  the  perfelic  order,  notwithstanding 
the  considerable  amount  of  leucite,  but  in  class  is  in  dosalane  close  to  the  border  of 
persalane,  in  rang  is  almost  exactly  on  the  border  between  the  domalkalic  and  alka- 
licalcic rangs,  and  in  subrang  is  dopotassic  but  very  close  to  the  sodipotassic  border. 
The  proper  symbol  for  its  position  then  would  be  I-II.  5.  2-3.  3-2,  and  the  name 
of  the  magma  corresponding  to  this  would  be  extremely  unwieldy.  It  seems 
best,  therefore,  to  indicate  in  the  magmatic  name  the  two  divisions  in  regard  to 
which  it  is  most  transitional,  so  that  the  name  of  ciminose-auruncose  has  been 
selected. 

Mode. — On  account  of  the  very  fine  grain  and  confused  arrangement  of  the 
groundmass  constituents  only  the  phenocrysts  could  be  estimated  optically.  Under 
these  circumstances  the  calculation  of  the  mode  from  the  norm  is  unsatisfactory 
and  must  be  based  on  several  assumptions  which  may  be  only  approximately  cor- 
rect. These  were  that  about  5  per  cent  of  leucite  is  contained  in  the  groundmass, 
that  the  composition  of  the  soda-lime  feldspar  phenocrysts  is  different  from  the 
feldspar  of  the  groundmass,  and  that  the  amount  of  olivine  is  about  2  per  cent. 
Making  these  assumptions,  and  with  due  regard  for  the  optically  estimated  relations, 
we  obtain  the  following  mode: 


Orthoclase,  Or,Ab! 22. 2 

Andesine,  Abr Ant 29 .  o 

Labradorite,  A^An^ 6.6 

Leucite 25 . 5 

Augite 9.1 


Olivine 2.3 

Ores 4.0 

Apatite 1.3 


While  this  mode  can  not  be  regarded  as  more  than  a  rough  approximation,  it  is 
probably  not  very  far  from  the  actual  facts.  Compared  with  the  norm,  the  mode 
differs  mainly  in  the  replacement  of  some  of  the  orthoclase  and  all  of  the  nephelite 
by  leucite  and  in  the  existence  of  augite  instead  of  olivine,  very  little  of  which  is 
present  modally.  Taking  these  into  consideration  the  type  may  be  called  an  augitic 
leucite-salphyro-ciminose-auruncose. 

Occurrence. — So  far  as  known  this  type  occurs  only  at  one  locality,  a  somewhat 
vesicular  flow  at  the  Croce  di  San  Martino,  on  the  northwest  crest  of  the  Vico  crater 
ring,  in  the  Ciminian  District. 

Name. — The  derivation  of  the  subrang  name,  auruncose,  will  be  explained  on 
a  later  page,  while  the  type  adjective  is  derived  from  Monte  Fogliano,  the  west- 
ern summit  of  the  Vico  Volcano. 


82  THE  ROMAN  COMAGMATIC  REGION. 

In  the  prevailing  systems  of  classification  the  rocks  of  this  type  would  best  be 
called  leucite-tephrite,  on  account  of  the  preponderance  of  labradorite  over  ortho- 
clase  among  the  phenocrysts,  as  weir  as  because  there  is  somewhat  more  soda-lime 
feldspar  than  alkali  feldspar  in  the  mode. 

POQLIANAL  CIMINOSE-AURUNCOSB.    II.  5.  2-3.  2. 

Megascopic  characters. — Dark  gray,  sprinkled  with  numerous  white  phenocrysts  of  leucite. 
Leucite  phenocrysts  very  abundant,  i  to  5  mm.,  white,  euhedral,  conspicuous.  Feldspar  pheno- 
crysts very  few,  small,  inconspicuous.  Augite  phenocrysts  few,  prismatic,  black,  inconspicu- 
ous. Groundmass  dark  gray,  aphanitic. 

Microscopic  characters. — Holocrystalline,  mediophyric,  dopatic.  Phenocrysts:  about  30  per 
cent,  leucite,  labradorite,  orthoclase,  augite.  Groundmass:  about  70  per  cent,  andesine,  ortho- 
clase,  augite,  leucite,  olivine,  magnetite,  apatite.  Fabric  felted  or  hyalopilitic  (  ?). 

Orthoclase,  OrjAbj. — Phenocrysts:  about  2  per  cent,  0.5  to  i.o  mm.,  subhedral,  stout 
prismatic,  inclusions  rare.  Groundmass:  about  20  per  cent,  part  anhedral  prismatic,  o.oi  to 
0.05  mm.,  part  as  interstitial  cement. 

Andesine,  AbtAn^ — Groundmass:  about  30  per  cent,  0.02  to  0.05  mm.,  anhedral,  part 
thin  prismatic,  part  apparently  as  interstitial  cement. 

Labradorite,  AbiAn2. — Phenocrysts:  about  7  per  cent,  0.5  to  2.0  mm.,  euhedral,  stout 
prismatic,  twinned,  inclusions  rare. 

Leucite. — Phenocrysts:  about  20  per  cent,  i  to  5  mm.,  euhedral  to  subhedral,  equant  trape- 
zohedra,  often  clustered,  inclusions  few.  Groundmass:  about  5  per  cent,  0.02  to  0.05  mm., 
anhedral,  equant,  inclusions  few. 

Augite. — Phenocrysts:  about  i  per  cent,  0.5  to  i.omm.,  subhedral,  stout  prismatic,  pale 
greenish  yellow,  nonpleochroic.  Groundmass:  about  8  per  cent,  0.005  to  0.02  mm.,  anhedral, 
prismatic  and  equant,  colorless. 

Olivine. — Groundmass:  about  2  per  cent,  o.i  to  0.5  mm.,  subhedral,  stout  prismatic  to 
equant,  colorless. 

Magnetite. — Groundmass:   about  4  per  cent,  0.005  to  o.oi  mm.,  anhedral,  equant. 

Glass. — Groundmass:  a  very  little  may  be  present,  but  is  not  easily  recognized. 

Chemical  composition  and  norm  as  on  p.  80. 

Type  specimen  from  Croce  di  San  Martino,  Vico  Volcano,  Ciminian  District. 

II.  5.  2-3.  2.    Teanal  Ciminose-Auruncose  [Leucite-Trachyte,  Teano  Type]. 

Megascopic  characters. — Rocks  of  this  type  are  rather  light  gray  and  porphy- 
ritic,  the  phenocrysts  of  leucite,  biotite,  and  augite  being  small  and  not  very  abundant. 
The  grayish  leucite  phenocrysts  vary  from  i  to  5  mm.  in  diameter,  are  sub- 
hedral to  anhedral,  and  not  very  conspicuous.  Those  of  the  alferric  minerals  are 
smaller,  seldom  over  2  mm.  in  length,  and  black,  the  biotite  in  thin  tables  and  the 
pyroxene  in  slender  prisms.  The  compact,  medium  gray  groundmass  is  aphanitic. 
In  the  field  these  rocks  would  be  called  biotite-leucite-leucophyres. 

Microscopic  characters. — In  thin  section  the  following  minerals  are  seen  to  be 
present:  Alkali  feldspar,  soda-lime  feldspar,  leucite,  augite,  biotite,  ores,  and  apatite. 
The  two  feldspars  are  present  in  about  equal  amount  and  together  make  up  about 
half  the  rock.  As  subhedral,  stout  prismoidal  phenocrysts  they  are  rare,  being 
for  the  most  part  in  the  groundmass  as  anhedral  patches  and  small  laths,  the  former 
mostly  of  the  alkali,  and  the  latter  of  the  soda-lime  feldspar,  though  it  is  not  always 
possible  to  distinguish  clearly  between  them,  except  by  the  refractive  indices,  on 


PETROGRAPHY. 


account  of  the  comparative  rarity  of  multiple  twinning  in  the  plagioclase.  The 
subhedral  leucite  phenocrysts  carry  few  inclusions  and  call  for  no  comment,  except 
that  they  are  not  very  abundant.  The  greater  part  of  this  mineral  is  in  the  ground- 
mass,  in  very  small,  spheroidal  anhedra,  these  and  the  phenocrysts  both  showing 
twinning  lamellae  with  the  selenite  plate.  The  augite  phenocrysts  and  the  ground- 
mass  prismoids  are  of  the  usual  variety,  subhedral  and  very  pale  gray  in  color,  with 
few  or  no  inclusions.  Biotite  exists  only  as  small  tabular  phenocrysts,  of  a  pale- 
brown  color,  pleochroic,  and  for  the  most  part  deeply  altered.  The  amount  of 
magnetite  is  small,  and  the  greater  part  is  found  as  small  anhedral  grains  scattered 
through  the  groundmass,  though  there  are  a  few  small  phenocrysts.  A  little  apatite 
is  present  in  small  prisms.  Olivine  is  wanting,  and  neither  nephelite  nor  glass  could 
be  detected. 

Chemical  Composition. — A  specimen  of  this  type  from  below  Orchi  was  chosen 
for  analysis,  as  it  seemed  to  be  fresher  than  those  from  near  Teano.  The  analysis 
has  not  hitherto  been  published. 

Chemical  Composition  of  Teanal  Ciminose-auruncose  [Leucite-frachyte], 


I. 

I. 

SiOj  

co.  86 

0.848 

CO2  

none 

ALO, 

18.48 

.l8l 

TiOj  

I    OO      O 

OT? 

Fe2O,  .  . 

4.03 

.O2C 

ZrO2  

O.O2 

FeO  

•J.4.C 

.048 

PjO...  . 

0.46 

OO7 

MeO.  . 

2.  5s 

.064 

so,... 

none 

CaO  .             

7.  77 

.  I3Q 

MnO  

nd. 

Na2O  

2.  23 

.O2C 

BaO  

0.  17 

K2O 

7.  1C 

.  O77 

H3O  +  

I.  Cl 

99.84 

H2O-  

o.  16 

Teanal  auruncose  [leucite-trachyte].     Below  Orchi,  Auruncan  District.     Washington,  analyst. 


Norm. 


Ratios. 


Or  

.  .4.2 

Ab  

6 

68.80] 

An        

.  .  10 

Ne  

6 

2C 

6.25  ) 

Di  

22  ) 

Ol  

i 

04  S 

14.26 

Mt  

.    e 

80  > 

n  .. 

08  S 

7.78 

Ap.. 

i 

02 

i  .02 

Rest  

98 
i 

II 
86 

Class. 


•  05 


23.06 


Order , 


Rang. . . 
Subrang , 


Sal 
^^  "  3-26 


1.62 


F 
'L 

K2O'  +  Na2O' 


CaO' 


K,O' 
'Na2O' 


99-97 


The  general  similarity  of  this  analysis  to  that  of  the  orvietal  type  next  to  be 
described  is  quite  close.  The  analysis  of  the  teanal  auruncose  shows  that  this 
type  is  well  within  the  borders  of  class,  order,  and  subrang,  but  so  close  to 


84 


THE  ROMAN  COMAGMATIC  REGION. 


the  border  of  the  domalkalic  rang  that  the  type  is  really  transitional  and  tht 
magma  a  ciminose-auruncose.  This  is  in  harmony  with  the  facts  noted  below 
that  the  orvietal  auruncose  would  be  termed  leucite-tephrite  in  prevailing  systems, 
on  account  of  the  more  prominent  labradorite,  while  the  teanal  type,  in  which  the 
soda-lime  feldspar  is  less  important,  would  rather  be  called  a  leucite-trachyte. 

Mode. — -The  mode  of  this  type  could  be  measured  microscopically  very  satis- 
factorily, except  that  the  groundmass  feldspars  had  to  be  lumped  together.  The 
readjustments  of  the  normative  molecules  to  calculate  the  mode  offered  no  points  of 
difficulty  or  special  interest,  the  measured  amount  of  leucite  being  assumed,  and 
otherwise  the  calculations  being  carried  out  as  in  previous  cases. 


CALCULATED. 


MEASURED. 


Orthoclase, 

Labradorite, 

Nephelite 

Leucite 

Augite 

Biotite 

Magnetite 

Apatite 


30.0 

24-5 
0.9 

15-7 
18.3 

2-3 

5-3 
i.o 


Vol.  %.      Sp.  gr.  Wt.  %. 

58.0  X   2.6   =  150.8     53.8 

17.3   X  2.5   =  43.3       15.4 

i?-2   X  3.3   =  56.8      20.3 

4.1X2.9=  11.9         4.2 

3.4   X  5.2   =  17.7        6.3 


IOO.O 


280.5     100.0 


The  two  agree  very  well,  though  the  calculations  show  that  a  very  small  amoun 
of  nephelite  is  probably  present,  which  might  easily  escape  detection.     The  erro 
due  to  overlapping  of  the  colored  minerals  is  noticeable,  but  not  serious.     The  cal 
culated  amount  of  biotite  is  that  derived  from  all  the  normative  olivine,  and  it  i 
notably  less  than  the  measured.    The  discrepancy  is  to  be  attributed  to  the  general 
and  profound  alteration  of  the  biotite  tables  to  a  fine-grained  aggregate  of  augite 
and  magnetite,  which  were  measured  with  the  unaltered  portions  as  biotite.     As 
compared  with  the  norm,  leucite  is  the  critical  mineral  and  biotite  the  varietal, 
and  as  the  fabric  of  the  groundmass  is  granular  rather  than  trachytic,  the  type  may 
be  described  as  biotitic  leucite-graniphyro-ciminose-auruncose. 

Occurrence. — The  only  known  occurrences  of  teanal  auruncose  are  in  the 
Auruncan  District,  where  it  seems  to  be  fairly  common.  Specimens  were  obtained 
in  the  southeastern  part  near  Teano,  as  at  Tuoro,  as  well  as  below  Orchi,  on  the 
northern  slope  of  the  volcano. 

Name. — As  mentioned  above,  the  subrang  name  is  derived  from  that  of  the 
district  where  it  seems  to  be  most  abundant.  That  of  the  type  comes  from  the 
town  of  Teano,  near  which  the  type  is  met  with. 

In  the  prevailing  classifications  these  rocks  could  be  called  either  leucite-teph- 
rites  or  leucite-trachytes,  but  as  the  alkali-feldspar  is  the  more  prominent,  the  name 
of  leucite-trachyte  would  seem  to  be  the  most  appropriate,  and  was  the  one  given 
the  type  in  a  former  description. 


PETROGRAPHY.  85 

TEANAL  CIMINOSE-AURUNCOSE.    II.  5.  2-3.  2. 

Megascopic  characters. — Medium  gray,  compact,  porphyritic.  Feldspar  and  leucite  phcno- 
crysts  few,  i  to  5  mm.,  white,  not  very  conspicuous.  Augite  phenocrysts  few,  i  to  2  mm.,  pris- 
matic, black.  Biotite  phenocrysts  few,  i  to  4  mm.,  thin  tabular.  Groundmass,  gray,  aphanitic. 

Microscopic  characters. — Holocrystalline,  mediophyric,  perpatic  to  dopatic.  Phenocrysts: 
about  15  per  cent,  leucite,  biotite,  orthoclase,  labradorite,  augite.  Groundmass:  about  85  per 
cent,  somewhat  interstitial  fabric,  orthoclase,  labradorite,  leucite,  augite,  magnetite,  apatite, 
nephelite. 

Orthoclase,  Or2Ab,. — Phenocrysts:  about  2  per  cent,  i  to  2  mm.,  subhedral,  stout  pris- 
matic, untwinned.  Groundmass:  about  28  per  cent,  0.05  to  0.20  mm.,  anhedral,  irregular 
patches,  sometimes  interstitial. 

Labradorite,  AbjAn2. — Phenocrysts:  about  2  per  cent,  i  to  2  mm.,  subhedral,  stout  pris- 
matic, twinned.  Groundmass:  about  22  per  cent,  0.02  to  o.  10  mm.,  subhedral,  thin  prismatic, 
twinned,  arrangement  diverse. 

Leucite. — Phenocrysts:  about  4  per  cent,  i  to  5  mm.,  subhedral,  equant,  inclusions  few. 
Groundmass:  about  12  percent,  0.05  to  o.  10  mm.,  anhedral,  spheroidal,  few  inclusions. 

Nephelite. — Groundmass:  about  i  per  cent,  anhedral,  interstitial,  difficult  to  detect. 

Augite. — Phenocrysts:  about  5  per  cent,  0.30  to  2.0  mm.,  subhedral  to  anhedral,  stout 
prismatic  to  equant  and  irregular,  pale  greenish-gray,  few  inclusions.  Groundmass:  about 
13  per  cent,  0.02  to  o.  10  mm.,  subhedral,  prismatic,  very  pale  greenish-gray. 

Biotite. — Phenocrysts:  about  4  per  cent,  i  to  4mm.,  subhedral,  thin  tabular,  brown, 
usually  much  altered.  Groundmass:  none. 

Magnetite. — Groundmass:    about  5  per  cent,  o.oi  to  0.02,  anhedral,  equant. 

Apatite. — Groundmass:   about  i  per  cent,  0.02  to  0.05  mm.,  subhedral,  prismatic. 

Chemical  composition  and  norm  as  on  p.  83. 

Type  specimen  from  below  Orchi,  Auruncan  District. 

II.  5.  3.  2.    Orvietal  Auruncose  [Leucite-Tephrite,  Orvieto  Type]. 

Megascopic  characters. — These  rocks  are  of  a  uniform  ash-gray  color,  and  differ 
from  those  yet  described  in  being  entirely  devoid  of  phenocrysts.  They  are,  how- 
ever, not  aphanitic,  since  the  mineral  particles  are  distinguishable  to  the  naked  eye, 
on  close  inspection,  as  white  and  dark,  the  former  predominating. 

Microscopic  characters. — Study  of  the  thin  sections  shows  that  micropheno- 
crysts  are  almost  entirely  wanting,  only  very  few  individuals  of  augite  and  an  excep- 
tional one  of  feldspar  attaining  dimensions  notably  greater  than  others,  and  the 
length  even  of  these  is  seldom  more  than  0.5  mm.  The  texture  shows  no  marked 
hiatus  in  the  sizes  of  the  particles,  and  the  extreme  limits  in  size  of  the  various 
minerals  are  approximately  the  same.  The  fabric  is  intersertal,  through  the  diverse 
arrangement  of  the  feldspar  laths. 

The  feldspar,  which  constitutes  about  50  per  cent  of  the  rock,  is  in  part  a  sodic 
orthoclase,  in  untwinned  anhedra,  up  to  0.4  mm.,  in  length.  The  borders  are  very 
irregular,  and  this  mineral  plays  the  r61e,  to  a  large  extent,  of  mesostasis  for  the 
other  constituents.  There  is  about  an  equal  amount  of  labradorite,  with  the  average 
composition  Ab2An3,  in  small  plates  and  laths,  which  exhibit  the  usual  multiple 
twinning.  Leucite  is  quite  abundant,  about  16  per  cent,  in  equant  anhedra  from 
0.05  to  0.2  mm.  in  diameter,  which  often  carry  a  few  inclusions  of  colorless  augite 
either  centrally  or  zonally  arranged.  The  augite,  which  forms  about  one-fifth  of 


86 


THE  ROMAN  COMAGMATIC  REGION. 


the  rock,  is  of  the  usual  very  pale  gray  and  for  the  most  part  in  subhedral  to  anhedral 
prismoids  of  about  the  same  size  as  the  leucites.     Brown  biotite  is  present  in  very 
subordinate  amount,  not  over  4  per  cent,  in  the  form  of  small,  irregular  patche 
between  the  subhedral  minerals,  not  that  of  tabular  phenocrysts.     About  the  same 
amount  of  magnetite  occurs  in  the  customary  small  grains.    A  little  olivine 
found  in  small  crystals,  and  there  is  a  very  little  interstitial  nephelite  cement, 
last  product  of  solidification. 

Chemical  composition. — An  analysis  was  made  of  a  specimen  from  the  massh 
flow  on  which  the  town  of  Toscanella  is  built,  taken  from  below  the  town  in 
ravine  of  the  Marta,  near  the  west  gate.     The  analysis  does  not  call  for  much 
comment  here.     It  will  be  noticed  that  the  silica  is  quite  low,  as  well  as  the  potash, 
while  the  lime  is  rather  high. 

Chemical  Compositon  of  Orvietal  Auruncose  [Leucite-tephrite\. 


I. 

I. 

SiO2  

51  .21 

o  85 

CO2  

none 

18.28 

.  I7Q 

TiO2  

I    A.1 

o  018 

FeaO3  

2..O7 

.OIO 

ZrO2  

FeO  

4.  io 

.o<;8 

P2O<  .  . 

°-35 

.00? 

MgO  

2.47 

.087 

SO,  .. 

none 

CaO  

7.86 

.  140 

s  

none 

Na2O  

2.4.O 

.040 

MnO  

n  d 

K2O  

6.60 

.070 

BaO  

o  io 

H~O4- 

H2O-   

o.  16 

99-77 

Orvietal  auruncose  [leucite-tephrite],  Toscanella,  Vulsinian  District.     Washington,  analyst. 

Norm.  Ratios. 

Or 38-92) 

Ab 9.17^67.27) 

An 19 . 18  )  f  73 • 66 

Ne 6.39      6.39  ) 


Mt.. 
II.  .. 
Ap.. 

Rest. 


Class  

Order  

Fern 
F 

"L 

Subranc  .  . 

CaO' 
KaO' 

2.91 


:io-53 


1.24 


a-74 

i .  02 

98.97 
0.82 

99-79 

Mode. — The  mode  of  the  Toscanella  rock  was  determined  both  by  Rosiwal's 
method  and  by  calculation  from  the  norm.  In  the  former  the  amounts  of  leucite 
and  biotite  could  be  measured  with  a  very  considerable  degree  of  exactness,  while 
the  two  feldspars  and  the  nephelite  were  necessarily  measured  together,  as  was  the 
case  with  the  augite  and  olivine.  In  calculating  the  mode  from  the  norm,  therefore, 
the  measured  amounts  of  leucite  and  of  biotite  were  assumed  to  be  correct,  as  some 
such  assumptions  were  necessary  to  distribute  the  potash  between  orthoclase,  leucite, 
and  biotite,  and  the  normative  olivine  molecules  between  modal  biotite  and  olivine. 


PETROGRAPHY.  87 

The  distribution  of  the  soda  was  fixed  by  the  composition  of  the  plagioclase 
and  the  use  of  the  usual  equations. 


CALCULATED. 

MEASURED. 

Orthoclase    Or2Abt          

26  o  ^ 

Vol.  %.      Sp.  gr. 

Wt.  %. 

Labradorite,  AbjAn?  

28.0  (• 

fK    n 

Nephelite  

I 
I.O  ] 

5°-9 

Leucite  

iq.  8 

if  H 

Augite         

16.8) 

15.0 

Olivine  

2.O  ) 

15-6    X   3.3    =      51.5 

18.8 

Biotite  

7.4 

Magnetit?  

6.0 

!  7   X  2  2   _       gg 

Apatite  

I.O 

IOO.O 

loo.o                      274.5 

IOO.O 

The  two  modes  resemble  each  other  very  closely,  except  that  in  the  calculated 
one  the  amount  of  magnetite  is  considerably  greater  than  in  the  measured.  As 
compared  with  the  norm,  the  only  difference  of  importance  is  the  modal  presence 
of  leucite,  which  replaces  much  of  the  orthoclase  and  changes  most  of  the  normative 
nephelite  to  albite.  The  readjustments  involved  by  the  presence  of  augite  and 
biotite  are  small  and  negligible.  The  type  may  therefore  be  described  as  leucite- 
grani-auruncose. 

Occurrence. — Orvietal  auruncose  is  a  rare  rock,  the  only  known  example 
forming  flows  near  Toscanella,  in  the  Vulsinian  District.  It  very  probably  occurs 
elsewhere  in  the  same  district,  and  possibly  in  the  Auruncan,  but  no  other  rocks  of 
similar  type  can,  as  yet,  be  definitely  referred  to  this  magma. 

Name. — The  name  of  the  subrang  is  derived  from  the  Auruncan  District, 
where  this  magma  seems  to  be  common,  though  usually  of  another  type.  The 
type  name  is  derived  from  the  city  of  Orvieto  in  the  Vulsinian  District,  near  which 
a  homologous  type  of  the  vicose  magma  is  met  with. 

In  the  prevailing  systems  of  classification  the  type  would  be  called  a  leucite- 
tephrite,  as  stress  is  laid  especially  on  the  soda-lime  feldspar. 

ORVIETAL  AURUNCOSE.    II.  5.  3.  2. 

Megascopic  characters. — Light  gray,  compact,  aphyric,  very  fine-grained,  phanerocrystalline. 

Microscopic  characters. — Holocrystalline,  serial  heterometric.  Orthoclase,  labradorite, 
leucite,  augite,  biotite,  olivine,  ores,  nephelite,  apatite.  Fabric  subintersertal. 

Orthoclase,  Or2Abj. — About  25  per  cent,  0.05  to  0.40  mm.,  anhedral,  irregular  patches, 
largely  intersertal,  untwinned. 

Labradorite,  Ab2An3. — About  28  per  cent,  o.io  to  0.40  mm.,  subhedral  to  anhedral, 
tabular,  twinned,  arrangement  diverse. 

Leucite. — About  16  per  cent,  0.05  to  0.20  mm.,  anhedral,  equant,  small  inclusions  of 
augite,  zonally  or  centrally  arranged. 

Nephelite. — About  i  per  cent,  anhedral,  interstitial  cement. 

Augite. — About  17  per  cent,  0.05  to  o.  20  mm.,  subhedral  to  anhedral,  prismatic  to  equant, 
pale  gray,  no  inclusions. 


88 


THE  ROMAN  COMAGMATIC  REGION. 


Biotite. — About  4  per  cent,  0.05  to  o.iomm.,  anhedral,  irregular  patches,  pale  brown, 
pleochroic. 

Magnetite. — About  4  per  cent,  0.02  to  0.05  mm.,  anhedral,  equant. 
Apatite. — About  i  per  cent,  0.05  to  o.io  mm.,  subhedral,  prismatic. 
Chemical  composition  and  norm  as  on  p.  86. 
Type  specimen  from  below  Toscanella,  Vulsinian  District. 

II.  5.  3.  3.    Monfinal  Shoshonose  [Biotite-Latite,  Monfina  Type]. 

Megascopic  characters. — This  rock,  which  is  only  met  with  in  the  region  as  a 
domal  eruption,  is  highly  porphyritic,  showing  many  small,  glistening,  black  tables 
of  biotite  and  small  greenish-black  augite  prisms,  with  numerous,  but  much  le 
conspicuous,  small,  stout  prisms  of  feldspar  in  a  very  fine-grained,  phanerocrystallinc 
groundmass,  which  is  very  light  gray,  but  with  a  slightly  pinkish  tone.  The  field 
name  for  the  type  would  be  biotite  leucophyre. 

Microscopic  characters. — The  most  abundant  phenocrysts  are  alferric,  of  a 
greenish-brown  biotite  in  thin  tables,  which  are  fresh  in  the  interior  but  with  a  thin 
border  of  alteration  products,  and  of  subhedral,  stout  prismoids  of  the  usual  augite, 
though  its  color  is  apt  to  be  slightly  more  green  than  in  the  types  just  described. 
Phenocrysts  of  feldspar  are  rather  less  numerous,  though  common,  and  consist  of 
both  labradorite  (Ab2An3)  and  soda-orthoclase.  These  are  subhedral,  stout  prisms, 
whose  average  lengths,  like  those  of  the  alferric  phenocrysts,  run  from  0.5  to  i  mm., 
seldom  being  greater  than  2  mm. 

The  groundmass  in  which  these  lie  is  very  largely  composed  of  slender  laths 
feldspar,  here  mostly  soda-orthoclase,  with  less  labradorite,  these  being  arrange 
fluidally,  so  that  the  fabric  is  a  typical  trachytic  one.  With  them  are  small  augite 
prismoids,  small  magnetite  anhedra,  and  very  few  apatite  needles.  All  these  lie  ii 
a  clear,  colorless  base,  which  is  not  isotropic,  but  which  everywhere  shows  a  distinct, 
though  somewhat  faint,  birefringence.  As  will  be  seen  later,  when  the  norm  and 
the  mode  are  discussed,  this  base  is  in  all  probability  either  quartz  or  a  mixture  of 
quartz  and  alkali-feldspar. 

Chemical  composition. — An  analysis  of  this  type,  made  several  years  ago,  is 
here  repeated  with  additional  determinations.  For  comparison  there  is  also  given 
an  analysis  of  another  type  of  shoshonose  from  Radicofani,  the  magma  of  which  is 
probably  connected  with  the  Vulsinian  District. 

Chemical  Composition  of  Monfinal  Shoshonose  [Biotite-latite]. 


I. 

II. 

I. 

II. 

SiOj  

C4.    c6       O    OOQ 

H2O  + 

O   17 

O.  1C 

A12O3  

17.87         .  I7C 

16.40          .  162 

CO2  

none 

none 

Fe2O3  

4O7         .02? 

I    02            006 

TiO2 

i  .02     o.  013 

i.io     0.014 

FeO  

326         o/ic 

S6c          078 

P-Oc 

O.  IO          .  OOI 

n.d. 

MgO  

3    /IT             08  C 

8C7              2TA 

MnO 

nd 

n.d. 

PaO 

Na20  

.»7          .123 
2.89          .047 

7-95       -142 

2.07     .033 

99-85 

100.91 

K2O  

4.41           .047 

3-35        -036 

So  er.  . 

2.  717 

PETROGRAPHY. 


NOTES  EXPLANATORY  OF  PRECEDING  TABLE. 

I.  Monfmal  shoshonose  [biotite-vulsinite].     Monte  Santa  Croce,  Auruncan  District.     Wash- 
ington, analyst.     Jour.  Geol.,  V,  1897,  p.  252. 

II.  Radicofanal  shoshonose  [andesite].  Radicofani,  Tuscany.     Washington,  analyst.     Am.  Jour. 
Sci.,  IX,  1900,  p.  52. 


o 

Norm  oj 
4 

* 

4.26  ' 

Or      

26 

Tl) 

Ab       

24 

fi-l  \- 

7^.28 

An  

22 

\ 

52  3 

Di  

22 

\ 

Hv   . 

.     C 

13  f 

13-95 

Mt  

.      C 

80  ) 

11  

I 

7.78 

An 

o 

-16 

O.  ^6 

Rest  

99 
o 

63 
17 

77-54 


22.09 


Class 


Ratios  of  I. 
Sal 


Order . 


Fern 

F 
Q 


Rang  .  . . 
Subrang . 


CaO' 


Na2O' 


99.80 


Or  

Norm  of  II 
20.02 

Ab  

17.  20 

An    .    ... 

.  2<;.8<; 

Di    

.     IO    8? 

Hy.  . 

.     21.27 

7-2  .  2Q 

Ol  

I  .  2? 

Mt  

I  .  1O 

11  

2    OO 

3-39 

Rest  

99.94 
O.  I? 

Class  

Ratios  of  II. 
Sal 

03.10 

Order  

Fern 
F 

36.78 

K^O'  +  N^O' 

Subrane  .  . 

CaO' 
K2O' 

=17.20 
=  1. 16 
•  i.oo 

=  1.71 
=    oo 
=  0.74 


100.09 

From  the  norm  of  I  it  is  clear  that  the  Santa  Croce  rock  is  well  within  the  bor- 
ders of  all  the  magmatic  divisions,  or,  in  other  words,  is  an  almost  central  shoshonose. 
The  Radicofani  rock,  on  the  other  hand,  which  carries  augite  and  very  little  olivine 
as  alferric  minerals,  but  no  biotite,  is  evidently  richer  in  these,  as  it  is  almost  in  the 
salfemane  class.  It  will  be  noted  also  that  the  monfinal  type  shows  an  excess  of 
silica,  appearing  as  normative  quartz,  while  the  radicofanal  has  deficient  silica, 
giving  rise  to  some  normative  olivine. 

Mode. — The  mode  of  this  type  could  not  be  determined  by  Rosiwal's  method, 
on  account  of  the  very  fine  grain  and  the  mineralogical  character  of  the  groundmass, 
though  the  relative  amounts  of  the  phenocrysts,  including  all  of  the  biotite,  were 
estimated  with  accuracy,  the  results  of  which  appear  in  the  formal  description. 
In  calculating  the  mode  from  the  norm  by  readjustment  the  observed  amount  of 
biotite  was  taken  as  a  starting-point.  That  this  is  probably  correct  is  shown  by  the 
fact  that  this  amount  takes  up  all  of  the  normative  hypersthene,  with  silica  lowered 
to  form  olivine  (3.84  per  cent),  which  with  an  almost  equal  amount  of  leucite  (3.5 
per  cent),  assumed  to  form  biotite,  makes  just  7.3  per  cent  of  this  mineral.  The 


90  THE  ROMAN  COMAGMATIC  REGION. 

silica  thus  set  free  in  the  formation  of  biotite  from  the  more  highly  silicated  minerals 
hypersthene  and  orthoclase,  having  nothing  to  combine  with,  must  be  present  as 
quartz  or  as  a  quaric  glass. 

Quartz 6.8 

Orthoclase,  Ora Abt 33-3 

Labradorite,  Ab2An3 34.  i 

Augite 12.  i 

Biotite 7.3 

Ores 6.0 

Apatite 0.4 


From  the  above  it  is  clear  that  a  certain  amount  of  quartz  must  be  present  in 
the  rock,  and  this  is  to  be  looked  for  in  the  rather  ill-defined,  colorless,  anisotropic, 
interstitial  base  already  mentioned.  The  feldspar  laths  and  what  are  clearly  anhedra 
of  orthoclase  are  so  closely  juxtaposed  that  study  of  this  base  is  difficult,  but  the 
mean  index  of  refraction  appears  to  lie  between  those  of  orthoclase  and  labradorite 
and  the  rock  does  not  furnish  any  gelatinous  silica  on  treatment  with  acid.  The 
mode  as  thus  calculated  corresponds  with  the  thin  sections  of  the  rock  as  well  as 
can  be  estimated,  and  may  be  considered  as  close  to  the  truth. 

The  variation  of  the  mode  from  the  norm  appears  chiefly  in  the  presence  of 
biotite,  the  other  readjustments  being  of  minor  importance.  The  type  may  there- 
fore be  described  as  a  biotite  trachiphyro-shoshonose. 

Occurrence. — This  type  is  a  decidedly  rare  one,  occurring  only  in  the  Auruncan 
District,  arid  here  only  at  the  central  dome  of  Monte  Santa  Croce,  which  is  com- 
posed apparently  entirely  of  it,  and  at  the  adjoining  small  hill  of  Monte  Lattani  in 
an  altered  condition. 

Name. — The  name  of  the  subrang  is  derived  from  that  of  Iddings's  group  of 
shoshonites,  many  of  which  fall  here.  The  type  name  is  derived  from  that  of  the 
volcano  of  Rocca  Monfina,  itself  so  called  from  the  name  of  the  village  at  the  foot 
of  Monte  Santa  Croce,  where  the  type  occurs. 

In  the  prevailing  systems  of  classification  the  type  has  been  assigned  to  very 
different  positions.  Called  a  trachydolerite  by  Abich,  as  far  back  as  1841,  it  is 
considered  by  Pilla,  vom  Rath,  and  Roth  to  be  a  trachyte,  while  Bucca  regards  it  as 
an  augite-andesite.  In  a  former  description  I  bestowed  the  name  of  biotite-vulsinite 
upon  it,  to  indicate  at  the  same  time  its  chemical  relation  to  the  vulsinites  and  its 
mineralogical  divergence;  and  this  would  seem  to  be  an  appropriate  name,  though 
it  would  best  be  called  a  biotite-latite. 

MONFINAL  SHOSHONOSE.    II.  5.  3.  3. 

Megascopic  characters. — Light  gray,  sometimes  with  pinkish  tinge,  compact,  porphyritic. 
Augite  phenocrysts  common,  black  or  greenish-black,  small  prisms.  Biotite  phenocrysts,  com- 
mon and  conspicuous,  black,  glistening,  small  tables.  Feldspar  phenocrysts  few,  small,  white, 
prismatic,  inconspicuous.  Groundmass,  fine-grained,  phanerocrystalline. 

Specific  gravity  =  2. 7 1 7. 


PETROGRAPHY.  91 

Microscopic  characters. — Holocrystalline  to  percrystalline,  mediophyric,''dopatic.  Pheno- 
crysts:  about  30  per  cent,  augite,  biotite,  labradorite,  orthoclase.  Groundmass,  about  70  per 
cent,  orthoclase,  labradorite,  augite,  magnetite,  apatite,  and  a  colorless  base,  sometimes  aniso- 
tropic,  occasionally  isotropic.  Fabric  trachytic. 

Orthoclase,  Or2Ab,. — Phenocrysts:  about  4  per  cent,  0.30  to  i  mm.,  euhedral  to  sub- 
hedral,  stout  prismatic,  sometimes  twinned,  inclusions  often  zonally  arranged,  few.  Ground- 
mass:  about  30  per  cent,  0.03  to  o.i  mm.,  subhedral  to  anhedral,  slender  prismatic  and 
interstitial. 

Labradorite,  Ab2An3. — Phenocrysts:  about  7  per  cent,  0.50  to  2.0  mm.,  euhedral  to  sub- 
hedral, stout  prismatic,  twinned,  inclusions  as  in  orthoclase.  Groundmass:  about  25  per  cent, 
0.03  to  o.iomm.,  subhedral  slender  prismatic,  twinned. 

Augite. — Phenocrysts:  about  8  per  cent,  0.5  to  2.0  mm.,  euhedral  to  subhedral,  stout 
prismatic,  pale  greenish.  Groundmass:  about  5  per  cent,  o.oi  to  0.05  mm.,  subhedral,  pris- 
matic, very  pale  greenish. 

Biotite. — Phenocrysts:  about  8  per  cent,  0.5  to  2-.o  mm.,  subhedral,  stout  tabular,  brown 
or  greenish  brown,  usually  altered  on  edges.  Groundmass:  none. 

Magnetite. — Groundmass:  about  5  per  cent,  o.oi  to  o.  10  mm.,  anhedral,  equant. 

Apatite. — Groundmass:  about  i  per  cent,  0.02  to  0.05  mm.,  subhedral,  slender  prismatic. 

Base. — Groundmass:  about  10  per  cent,  colorless,  usually  indefinitely  anisotropic  or 
microfelsitic,  sometimes  isotropic,  probably  composed  of  quartz  and  orthoclase,  does  not  gela- 
tinize with  acids. 

Chemical  composition  and  norm  as  on  p.  88. 

Type  specimen  from  Monte  Santa  Croce,  Auruncan  District. 

II.  6.  2.  2.    Poglianal  Vicose  [Leucite-Tephrite,  Viterbo  Type]. 

Megascopic  characters. — This  type  in  the  hand  specimen  resembles  the  viterbal 
vulsinose  and  ciminose  previously  described  so  closely  that  it  is  quite  impossible  to 
distinguish  them  in  the  field.  It  shows  the  same  characteristic  viterboid  habit, 
very  abundant  large  phenocrysts  of  leucite  in  a  light  gray  groundmass.  The  leu- 
cite  phenocrysts  are  from  5  to  20  mm.  in  diameter  as  a  rule,  but  may  run  up  to  50 
mm.,  and  are  highly  euhedral  in  well-formed  trapezohedra,  sometimes  fragmentary, 
either  white  or  grayish,  and  often  showing  small  dark  inclusions.  With  these  are  few 
small  black  prismatic  phenocrysts  of  augite,  with  still  more  rare  prismatic  pheno- 
crysts of  feldspar.  The  groundmass  is  of  a  rather  light  gray,  and  is  either  quite 
aphanitic  or  very  fine-grained  phanerocrystalline,  only  the  distinction  between  the 
light  and  dark  minerals  being  evident. 

Microscopic  characters. — In  the  thin  section  the  large  leucite  phenocrysts 
exhibit  the  double  refraction  and  twinned  structure  very  clearly,  and  the  rather 
numerous  inclusions  are  seen  to  be  of  augite,  magnetite,  labradorite,  and  glass, 
commonly  clustered  toward  the  center.  The  few  feldspar  megaphenocrysts  visible 
are  mostly  of  a  labradorite  of  about  AbIAn2,  in  stout  subhedral  prisms,  much 
twinned,  while  the  phenocrysts  of  augite  are  in  stout,  subhedral  prismoids,  from 
0.5  to  i  mm.  long,  often  fragmentary,  and  of  the  usual  pale-gray  or  slightly  greenish 
color. 

The  groundmass  in  which  these  lie  is  holocrystalline  typically,  though  a  little 
glass  base  may  be  present  in  some  cases.  The  fabric  varies  somewhat,  but  is  usually 


92 


THE  ROMAN  COMAGMATIC  REGION. 


rather  granular,  in  some  specimens  rather  felted,  and  its  grain  is  also  variable,  the 
more  granular  fabrics  being  found  in  the  groundmasses  with  the  coarser  grain. 
Microphenocrysts  of  leucite  are  common  in  the  groundmass,  in  anhedral,  usuallj 
rounded  and  occasionally  irregular  individuals,  which  show  but  faint  double  reft 
tion  and  carry  few  inclusions.  Microphenocrysts  of  feldspar  are  quite  common. 
These  are  soda-orthoclase,  which  sometimes  shows  a  moire*  appearance,  and  of  lab- 
radorite.  The  relative  amounts  of  these  vary  in  different  specimens;  in  some  cases 
those  of  alkali-feldspar  surpassing  those  of  labradorite,  as  in  the  specimen  from 
Monte  Fogliano,  while  again  the  reverse  is  true,  as  in  the  rock  from  Monte  San 
Antonio.  The  micro-groundmass  shows  numerous,  anhedral  prismoids  and  grains 
of  augite  and  very  few  small,  irregular  grains  of  olivine.  It  is  composed  in  large 
part  of  feldspar,  apparently  almost  entirely  a  soda-orthoclase,  which  is  sometimes 
in  small  laths  with  a  felted  fabric,  and  again  in  formless  anhedra,  both  forms  occur 
ring  together  as  well.  There  are  the  usual  small  magnetite  grains  and  some  smal 
apatite  needles.  A  small  amount  of  interstitial  nephelite  is  seen  in  some  specimens 
which  is  replaced  in  others  by  an  equally  small  amount  of  glass  base  which  is  deter 
minable  with  difficulty. 

Chemical  composition. — Two  analyses  were  made  of  this  type,  neither  of  whid 
has  been  published  hitherto.  With  them  in  the  table  is  repeated  for  comparisor 
the  analysis  of  the  viterbal  vulsinose  from  the  Vico  Volcano. 

Chemical  Composition  of  Foglianal  Vicose  [Leucite-tephrite]. 


I. 

II. 

III. 

SiO2  

54.  8* 

O.QI4. 

51  .  20     o. 

8?*, 

e6.  IQ 

AlaO3  

ID  .  ?O 

.  IO2 

21.21 

208 

2O.  7? 

FeaO3  

1.66 

.  OIO 

2.  ?8 

QIC 

1  .  71 

FeO  

3.  O4 

.  O4.2 

•5  .67 

OCX 

2  .  IO 

MgO  

1.40 

•  O37 

I  -00 

(XO 

1  .  14 

CaO  

A..  O? 

.072 

c  .42 

006 

7.  C2 

Na2O  

2  .  O2 

.  O47 

2  .  II 

034 

2.86 

K2O  

TO    AQ 

•  III 

IO.  67 

112 

TO.  47 

H2O+  

O.  77 

0.28 

O.  7O 

H2O-  

O.4Q 

O.  IO 

O.  TO 

CO2  

none 

none 

none 

TiO2  

O.  7? 

.  ooo 

O.  74 

ooo 

O.61? 

ZrO2  

o.  01 

O.O3 

P,Oe.. 

o.  17 

.  OOI 

o.  36 

OO3 

O    24 

SO3  

none 

trace 

MnO  

n.d. 

n.d. 

n.d. 

BaO  

O.  I? 

O.  33 

100.30 

100.45 

100.73 

I.  Foglianal  vicose   [leucite-tephrite].      Monte  Fogliano,  Vico  Volcano,  Ciminian  District. 

Washington,  analyst. 
II.  Foglianal  vicose  [leucite-tephrite].     Monte  San  Antonio,  Auruncan  District.     Washington, 

analyst. 

III.  Viterbal   vulsinose   [leucite-trachyte].     Sorgente    di    Grignano,  Vico  Volcano,   Ciminian 
District.     Washington,  analyst. 


PETROGRAPHY. 


93 


•:•! 

Or   .. 

Vorm  oj  I 
.  61  .  72  ^ 

Ab  

O.  52  f 

71  -60 

An   ..  . 

3'  f 
n  .4.C   > 

Lc  

Ne  

.     13.06 

1  3.06 

Di    .  .  . 

7.08  ) 

Ol  .  .  .  . 
Mt.... 
11 

•     i-97) 
2.32  ) 

I     C2   1 

9-95 
3-84 

Ap.... 

•       0.34 

o-34 

Rest  .  .  . 

98.88 
.       1.42 

Norm  oj  II. 
37-53 


14-75 


14-13 


100.30 


16.96 

19.84 


3-48 

i-37 
0.92 

99.67 
Q-74 

100.41 


54-49 


9.91 

4-85 
0.92 


83-99 


15.68 


Ratios. 


II. 


Class  

Sal 

=  6.  oo 

576 

Order  

Fern 
F 

=  ?  .  4.Q 

I  84 

"I, 

—  —  A  6c 

CaO' 
K2O' 

Na2O' 

The  analyses  are  remarkable  for  their  figures  for  potash,  which  are  among  the 
highest  recorded  for  this  constituent.  Those  of  foglianal  vicose  resemble  each  other 
closely,  the  chief  difference  being  in  the  silica.  The  analyses  in  I  and  III  are  mark- 
edly similar,  the  percentages  of  potash,  soda,  and  ferric  and  titanic  oxides  being  almost 
duplicates,  while  those  of  ferrous  oxide,  magnesia,  and  lime  do  not  differ  much. 
But  in  I  both  silica  and  alumina  are  lower  by  rather  more  than  i  per  cent,  and  it  is 
due  to  this,  as  well  as  to  the  fact  that  the  differences  in  other  constituents,  while 
slight,  all  work  out  in  the  same  classificatory  direction,  that  the  rock  represented  by 
I  falls  in  the  dosalane  class  and  the  lendofelic  order.  Examination  of  the  norms 
and  ratios  shows  that  both  of  the  specimens  of  vicose  fall  well  within  all  the  classifi- 
catory divisions,  though  II  is  rather  close  to  the  border  of  the  lenfelic  order,  but  not 
enough  so  to  be  considered  transitional. 

Mode. — In  neither  case  could  the  mode  be  determined  satisfactorily  by  Rosi- 
wal's  method,  owing  to  the  fine  grain,  the  fabric,  and  the  mineral  composition  of 
the  microgroundmass,  though  the  relative  amounts  of  the  phenocrysts  could  be 
easily  estimated,  with  the  results  found  in  the  formal  description.  On  the  other 
hand,  the  mode  could  be  calculated  from  the  norm  with  ease,  the  usual  procedure 
being  followed.  The  results  of  these  calculations  of  the  two  rocks  are: 


i.  ii. 

Soda-orthoclase,  OrjAbj. .   32.0  13.5 

Labradorite,  AbjAnj n.6  23.8 

Leucite 40 . 6  44 . 2 

Nephelite none  i .  7 

Augite ii. 2  7.9 


I.  II. 

Olivine 2.0  4.2 

Magnetite 2.2  3.8 

Apatite 0.3  0.9 

IOO.O    IOO.O 


94  THE  ROMAN  COMAGMATIC  REGION. 

The  two  modes  are  closely  alike  in  all  respects,  except  as  to  the  relative  amount 
of  the  two  feldspars.     Indeed,  the  divergence  is  so  great  that  they  should  strictly 
regarded  as  two  distinct  types.     But  considerations  of  the  inadvisability  of  making 
the  types  too  numerous  at  this  stage  has  resulted  in  the  final  decision  to  throw  ther 
both  together,  leaving  their  separation  to  the  future  if  the  need  for  this  is  felt.    It 
is,  however,  to  be  noted  that  the  difference  in  mode  shown  above  is  evident  in  the 
thin  sections,  those  of  the  Auruncan  rock  (II)  showing  more  abundant  labradorite 
than  the  other. 

The  composition  of  the  alkali-feldspar  is  a  striking  feature,  the  albite  molecule 
in  each  case  being  double  that  of  the  orthoclase.  This  is  a  decidedly  unusual  com- 
position for  the  alkali-feldspars  of  the  region,  which  usually  have  the  orthoclase 
molecule  predominant  over  that  of  albite.  It  is  difficult  to  check  this  calculatec 
composition  by  optical  means,  on  account  of  the  small  size  of  the  groundmass  laths 
the  absence  of  twinning,  and  the  frequency  of  formless  anhedra,  but  such  measure 
ments  as  were  made  would  seem  to  confirm  it,  and  it  will  be  remembered  that  the 
larger  alkali-feldspars  in  these  rocks  show  the  moire*  appearance  which  is  unusua 
elsewhere  in  this  region. 

As  compared  with  the  norm,  the  only  difference  of  importance  is  the  replace- 
ment of  normative  nephelite  by  modal  leucite,  and  the  consequent  changes  in  the 
figures  for  orthoclase  and  albite.  The  type  may  then  be  described  as  leucite  sal- 
phyro-vicose. 

Occurrence. — This  type  is  a  rather  common  one,  being  found  in  the  four  larges 
districts.  In  the  Vulsinian  District  the  most  prominent  locality  is  a  flow  above 
Santa  Trinit£,  near  Orvieto,  but  not  the  one  whence  came  the  specimen  describee 
by  Klein.*  In  the  Ciminian  District  the  type  is  especially  abundant,  along  with 
the  homologous  types  of  vulsinose  and  ciminose,  among  the  products  of  the  Vico 
Volcano,  as  in  the  inner  north  wall,  the  Contrada  di  Merlano  and  the  Villa  di 
Buonrespiro  on  the  northwestern  slope,  and  as  flows  from  Monte  Fogliano  on  the 
western.  In  the  Sabatinian  District  it  was  found  at  Lagosello,  northeast  of  Lake 
Bracciano.  In  the  Auruncan  District  it  is  quite  common,  specimens  having  been 
obtained  by  me  from  Monte  San  Antonio,  on  the  north  wall,  and  occurring  as  well 
probably  at  Colle  Friello,  Fontanelle,  and  above  San  Martino,  to  judge  from  the 
descriptions  of  Bucca. 

Name. — The  name  of  the  subrang  is  derived  from  that  of  the  Vico  Volcano, 
where  the  magma  is  quite  abundant  and  where  one  of  the  most  prominent  types 
occurs.  The  type  adjective  is  derived  from  the  name  of  the  highest  point  of  the 
Vico  crater,  Monte  Fogliano,  where  the  type  occurs. 

In  the  prevailing  systems  this  type  has  been  very  variously  classified,  accord- 
ing to  the  personal  bias  as  to  the  relative  importance  of  the  salic  minerals,  the  names 
leucite-trachyte,  leucite-tephrite,  leucite-phonolite,  and  leucitophyre  having  been 
bestowed  upon  it  by  different  authors,  and  the  name  of  leucite-basanite  being  also 

*  Klein,  Neu.  Jahrb.,  B.  B.  VI,  1889,  p.  ic>. 


PETROGRAPHY. 


95 


of  possible  application,  on  account  of  the  olivine  present.  But  the  amounts  of 
nephelite  and  olivine  are  so  small  as  to  be  quite  negligible  in  any  system  which 
rationally  takes  into  account  the  relative  amounts  of  the  minerals,  and  the  choice 
must  depend  on  the  relative  amounts  of,  and  the  importance  attributed  to,  the 
soda-orthoclase  and  the  labradorite.  It  has  been  shown  above  that  in  two  speci- 
mens the  relations  of  these  are  inverse,  and  that  it  might  be  advisable  to  distinguish 
two  types,  which  would  find  expression  in  the  prevailing  systems  in  the  names  leucite- 
trachyte  and  leucite-tephrite.  But  in  view  of  the  facts  that  the  soda-lime  feldspar 
is  rather  more  prominent,  even  in  the  rock  in  which  it  is  subordinate  in  amount, 
the  usual  greater  importance  given  to  this  feldspar,  and  the  peculiar  composition 
of  the  alkali-feldspar,  which  approaches  an  albite  in  composition,  the  name  of 
leucite-tephrite  would  seem  to  be  the  most  appropriate  for  the  rock.  Were  a  new 
name  desirable  for  this  type,  which  so  much  resembles  the  viterbal  vulsinose  and 
ciminose,  that  of  vicoite  would  be  appropriate. 

FOQLIANAL  VICOSE.    II.  6.  2.  2. 

Megascopic  Characters. — Light  gray,  compact,  highly  porphyritic.  Leucite  phenocrysts 
very  abundant,  almost  one-half  of  rock,  5  to  20  mm.,  or  larger,  euhedral  trapezohedra,  some- 
times fragmentary,  white  to  very  pale  gray.  Feldspar  phenocrysts  very  few,  2  to  20  mm.,  stout 
prismatic,  colorless.  Augite  phenocrysts  very  few,  i  to  2  mm.,  prismatic,  black.  Groundmass: 
light  gray,  aphanitic  or  rarely  very  fine-grained  phanerocrystalline. 

Microscopic  characters. — Percrystalline,  magnophyric,  dopatic.  Phenocrysts :  35  per  cent, 
leucite,  labradorite,  augite,  sometimes  orthoclase.  Groundmass:  65  per  cent,  microporphy- 
ritic,  granular,  sometimes  partly  felted  fabric,  orthoclase,  labradorite,  leucite,  augite,  olivine, 
magnetite,  apatite,  sometimes  nepheh'te  or  glass  base. 

Soda-orthoclase,  OrjAbj  to  OrtAb2. — Phenocrysts:  about  2  per  cent,  not  always  present, 
5  to  20  mm.,  subhedral,  stout  prismoidal,  Carlsbad  twinning,  few  inclusions.  Groundmass: 
15  to  30  per  cent,  0.02  to  o.  20  mm.,  subhedral  to  anhedral,  in  part  tabular,  in  part  as  interstitial 
areas. 

Labradorite,  AbtAnj. — Megaphenocrysts:  about  3  per  cent,  5  to  20  mm.,  subhedral,  stout 
prismoidal,  twinned,  inclusions  few.  Microphenocrysts:  from  5  to  20  per  cent,  o.  i  to  0.5  mm., 
subhedral  to  anhedral,  thick  tabular,  twinned.  Microgroundmass:  about  5  per  cent,  not 
always  present,  0.02  to  o.o.  10  mm.,  anhedral,  tabular. 

Leucite. — Megaphenocrysts:  about  30  per  cent,  5  to  20  mm.  or  more,  euhedral,  equant 
trapezohedra,  often  fragmentary,  twinned,  inclusions  common  of  augite,  labradorite,  magnetite, 
and  glass,  usually  centrally  arranged.  Microphenocrysts:  about  10  per  cent,  0.05  to  0.50  mm. 
subhedral  to  anhedral,  equant  spheroidal,  sometimes  irregular,  faint  twinning,  inclusions  few. 

Nephelite. — Groundmass:   5  per  cent  to  none,  anhedral,  formless,  interstitial  areas. 

Augite. — Phenocrysts:  about  2  per  cent,  0.5  to  2.0  mm.,  subhedral,  stout  prismoidal,  gray 
to  pale  greenish-yellow,  non-pleochroic,  inclusions  rare.  Groundmass:  about  10  per  cent,  0.05 
to  o.  20  mm.,  subhedral  to  anhedral,  prismoidal  to  equant  and  irregular,  colorless  or  pale  gray. 

Olivine. — Groundmass:    about  3  per  cent,  0.02  to  o.iomm.,  anhedral,  equant,  colorless. 

Magnetite. — Groundmass:  about  2  per  cent,  0.02  to  0.05  mm.,  anhedral,  equant. 

Apatite. — Groundmass:   about  i  percent,  o.oi  to  0.05  mm.,  subhedral,  thin  prismoidal. 

Glass. — Usually  none,  sometimes  up  to  5  per  cent,  colorless,  difficult  to  detect. 

Chemical  composition  and  norm  as  on  p.  92. 

Type  specimens  from  Convento  di  Sant'Angelo,  Monte  Fogliano,  Vico  Volcano,  Ciminian 
District,  and  from  Monte  San  Antonio,  Auruncan  District. 


96  THE  ROMAN  COMAGMATIC  REGION. 

II.  6.  2.  2.    Bagnoreal  Vicose  [Leucite-Tephrite,  Bagnorea  Type] . 

Megascopic  characters. — In  the  hand  specimen  these  rocks  closely  resemble 
the  bagnoreal  ciminose  already  described.  They  are  medium  gray,  varying  some- 
what in  different  specimens,  but  never  becoming  very  light  or  very  dark.  While 
distinctly  porphyritic,  phenocrysts  are  few  and  inconspicuous.  These  are  mostly 
of  leucite,  in  subhedral  crystals,  from  3  to  10  mm.  in  diameter,  usually  of  a  very 
pale-gray  color,  often  with  a  slightly  yellowish  tinge.  Small  prismatic  phenocrysts 
of  black  augite  are  still  more  rare.  In  some  specimens  are  one  or  two  rounded 
patches  (up  to  5  cm.  in  diameter),  of  medium-grained  aggregates  of  small  leucite 
and  augite  grains,  apparently  early  solidified  segregations  from  the  magma  (enclaves 
hotnceogenes).  The  groundmass  is  distinctly  phanerocrystalline  and  fine-grained, 
the  separate  particles  of  light  and  dark  minerals  clearly  visible  to  the  naked  eye. 

Microscopic  characters. — Studied  in  thin  section  the  rare  leucite  phenocrysts 
show  more  or  less  irregular  outlines,  the  crystal  form  not  having  been  well  developed. 
The  double  refraction  and  twinning  are  strongly  marked,  and  inclusions  are  not 
common.  The  subhedrally  prismatic  augite  phenocrysts  are  of  the  usual  pale-gray 
color,  though  in  some  specimens  there  is  a  tendency  to  zonal  growth,  as  shown  by  a 
slight  deepening  of  the  greenish  tint  toward  the  center  or  by  progressive  extinctions. 

The  groundmass  is  holocrystalline  in  nearly  all  the  specimens  examined,  the 
fabric  is  intersertal,  and  it  is  composed  in  great  part  of  leucite,  partly  in  the  form  of 
small,  subhedral  spheroids,  and  partly  in  quite  irregular  anhedra.  All  these  are 
clear  and  carry  few  if  any  inclusions.  With  these  leucites  are  many  small  feldspar 
laths,  the  majority  being  of  a  twinned  labradorite  about  Ab2An3,  with  fewer  of 
alkali-feldspar,  which  is  also  present  to  some  extent  as  an  interstitial  cement  in 
patches.  The  arrangement  of  these  feldspar  laths  is  diverse,  producing  an  intersertal 
fabric.  Small  augites  are  also  abundant  in  stout  subhedral  prismoids,  often  so  short 
as  to  be  almost  equant.  The  gray  color  of  this  is  also  slightly  more  greenish  than 
is  usually  true  of  the  rocks  of  the  region,  though  the  absence  of  pleochroism  indicates 
that  it  does  not  carry  any  of  the  aegirite  molecule.  Small  magnetite  grains  and 
apatite  prisms  are  present,  both  in  rather  greater  amount  than  is  usually  the  case 
in  similar  rocks  elsewhere  in  the  region,  but  no  olivine  could  be  detected.  In  some 
of  the  specimens  there  is  some  colorless  nephelite  as  an  interstitial  base,  the  last 
product  of  solidification,  while  in  a  few  this  is  partly  replaced  by  a  colorless  glass,  the 
amount  of  which,  however,  is  never  great. 

Chemical  composition. — Of  this  type  two  hitherto  unpublished  analyses  are  given. 
In  III  is  found  an  earlier  analysis  of  the  same  specimen  as  that  used  for  II,  in  which 
MgO  was  determined  from  loss,  and  with  no  determinations  of  TiO2  and  P2OS. 
As  this  was  unsatisfactory,  all  the  main  constituents  from  silica  to  lime  were  rede- 
termined  on  another  portion  of  the  specimen,  the  determinations  of  the  alkalis  being 
accepted  as  correct;  and  these,  with  the  additional  determinations  of  the  minor 
constituents,  form  the  analysis  in  II. 


PETROGRAPHY. 

Chemical  Composition  of  Bagnoreal  Vicose  [Leucite-tephrtie]. 


97 


I. 


II. 


III. 


Ala03. 
Fea03. 
FeO  .. 


HaO- 
TioY.' 


PaOs. 
SO3... 

S 

MnO.. 

CuO. 

BaO.. 


50.68 
19.46 
3-96 
2-5i 
2.24 
6.78 
2.61 
9-38 
0.46 
o.  16 
none 
0.89 
trace 

o-33 
none 
none 
n.d. 
none 
o-i5 
99.61 


0.845 
.191 
.025 

•035 
.056 

.  121 
.042 
.  IOO 


50.36 

17.62 

4.80 

2-53 
3-27 
7.61 
1.99 
9-39 


1.09 
n.d. 

0.40 


n.d. 
n.d. 
n.d. 

100.25 


0.839 

•173 
•  030 

•035 
.082 

.136 
.032 

.  IOO 


.014 


.003 


49-73 
19.20 

5-50 
2.41 


g6 
95 


9-39 
1.19 

n.d. 
n.d. 

n.d. 


n.d. 


Sp. 


2-655 


2-655 


*  MgO  by  difference. 

I.  Bagnoreal  vicose  [leucite-tephrite].     Poggio  Cotognola,  near  Bracciano,  Sabatinian  Dis- 
trict.    Washington,  analyst. 
II.  Bagnoreal  vicose  [leueite-tephrite].     Madonna  del   Riposo,    below    Bracciano,  Sabatinian 

District.     Washington,  analyst. 

III.  Bagnoreal  vicose  [leucite-tephrite].     Madonna  del   Riposo,  below    Bracciano,   Sabatinian 
District.     Washington,  analyst.     Jour.  Geol.,  V,  1897,  p.  49. 

Norm  of  I.  Norm  of  II. 

9r 39'f!  53-10)  35-03  |    6        ) 

An 13.62$°-         (776?  4'40?  f7i6<; 

Lc 12.62  (77  -05  16.13)        22C71- 

Ne 11.93)        ->•>  9-09  \    - 

Di 12.10)    ,  I7-7Il-rK^f. 

r  I  2  .  1 4  f  I  o . OO 

Wo 1.04$  *3***  0.35  } 

n':::::::::  f:£f  ,.„  «••>    1:31 ».«  2746 

Hm o.oo  )  1.44) 

Ap 0.73       0.73  0-96      0.96 

98.84  99-n 

Rest 0.77  1.19 

99.61  100.30 

Ratios.  I.        II. 

Sal 

Class ^ —  =3-66     2.61 

rem 

Order £  =2-16     1.84 

Ra*g KaO^+NaaO  =2  ^    3  22 

Subran8 N^O7  =2'38     3'13 


98  THE  ROMAN  COMAGMATIC  REGION. 

The  two  analyses  I  and  II  are  very  closely  alike,  especially  in  silica,  ferrous 
oxide,  and  potash.  The  alumina  in  I  is  the  higher  by  2  per  cent,  and  as  this  is 
accompanied  by  i  per  cent  less  of  magnesia,  the  suspicion  may  present  itself  that 
some  of  the  magnesia  has  been  thrown  down  with  the  alumina.  This  analysis, 
however,  is  the  later  of  the  two,  and  especial  attention  was  paid  at  the  time  to  this 
very  point.  Furthermore,  it  will  be  observed  that  the  general  character  of  I  in  other 
respects  is  more  salic  than  II,  as  shown  by  the  slightly  higher  soda  and  lower 
ferric  oxide,  lime,  titanium,  and  phosphoric  oxide;  and  these  chemical  differences 
are  in  harmony  with  the  rocks  themselves.  The  Cotognola  rock  (I)  is  very  notably 
lighter  in  color,  and  the  sections  show  somewhat  less  alferric  and  femic  minerals 
than  that  from  Madonna  del  Riposo  (II). 

The  analyses  in  themselves  call  for  no  special  comment.  Silica  is  lower  than 
in  any  rock  so  far  described  here,  and  while  potash  is  high,  it  is  distinctly  less  so 
than  in  any  previously  described  types  of  vicose.  The  magmatic  positions  shown 
by  the  norm  are  in  most  cases  well  within  the  limits,  except  the  ordinal  position  of 
II,  which  approaches  rather  near  that  of  the  lenfelic  order,  but  neither  rock  can  be 
justly  regarded  as  transitional. 

On  the  basis  of  the  analysis  shown  in  III  the  rock  from  Madonna  del  Riposo 
has  been  assigned*  to  the  subrang  braccianose,  II.  7.  2.  2.  This  was  correct  as 
based  on  the  analytical  data  then  available,  but  the  new  analysis  shows  that  the  true 
position  is  rather  in  vicose,  II.  6.  2.  2,  though,  as  we  have  seen  above,  somewhat 
near  the  border  of  the  division  to  which  it  was  first  assigned.  The  change  is  brought 
about  by  several  factors,  each  of  slight  moment  in  itself,  but  all  acting  in  the  same 
direction,  the  increase  in  the  figures  for  silica  and  magnesia,  the  decrease  in  alumina 
and  lime,  and  the  additional  determinations  of  titanium  and  phosphoric  oxides. 

Mode. — The  mode  of  the  Cotognola  rock  was  calculated  from  the  norm  by 
readjustment  of  the  mineral  molecules,  in  the  usual  way.  It  was  somewhat  difficult 
to  distinguish  the  colorless  groundmass  minerals  with  sufficient  accuracy,  so  that 
the  Rosiwal  method  was  not  applied,  though  a  rough  estimate  of  the  leucite  present 
was  undertaken  to  use  in  the  calculation.  This  amounted  to  40  per  cent  by  volume. 
As  the  Riposo  rock  is  not  holocrystalline,  containing  about  5  per  cent  of  glass,  its 
mode  was  not  calculated,  but  it  will  resemble  the  other  closely. 

Orthoclase,  Or^Abi 17.5 

Labradorite,  AbrAn2 16. 8 

Leucite 37.1 

Nephelite 4.6 

Augite 18. 6 

Magnetite 4.4 

Apatite i .  o 

100.0 

This  mode  shows  the  distinctly  more  alferric  character  of  the  type,  indicated  in 
the  analysis  by  the  lower  silica  and  the  higher  bivalent  metal  oxides.  The  amounts 

*  Washington,  Prof.  Paper  U.  S.  Geol.  Surv.  No.  14,  1903,  p.  305. 


PETROGRAPHY. 


99 


of  alkali  and  soda-lime  feldspars  are  almost  identical,  leading  to  the  uncertainty  as 
to  the  classification  under  prevailing  systems,  though  the  difference  in  habit  and 
prominence  of  the  two  in  the  groundmass  reduces  this  to  some  extent. 

As  in  so  many  of  the  preceding  cases,  the  modal  divergence  from  the  norm  is 
brought  about  largely  by  the  increase  in  leucite  and  decrease  in  orthoclase  and 
nephelite,  making  leucite  the  critical  mineral.  The  type  may  therefore  be  described, 
as  in  the  case  of  the  two  preceding,  as  leucite  salphyro-vicose. 

Occurrence. — This  type  seems  to  be  most  abundant  in  the  Sabatinian  District, 
typical  localities  being  flows  at  Madonna  del  Riposo,  below  the  town  of  Bracciano, 
and  at  a  quarry  along  the  railroad  below  Poggio  Cotognola,  southwest  of  Bracciano. 
Some  flows  at  Monte  Bisenzo,  on  the  south  shore  of  Lake  Bolsena,  in  the  Vulsinian 
District,  may  also  be  referred  here,  and  undoubtedly  belong  to  the  subrang  of 
vicose,  though  most  of  them  are  so  poor  in  phenocrysts  as  to  belong  rather  to  the 
aphyric  orvietal  type  to  be  described  later. 

Name. — Both  the  subrang  and  type  names  have  already  been  discussed.  In 
the  prevailing  classifications  this  type  must  be  considered  as  a  leucite-tephrite,  in 
view  of  the  mode  and  texture,  since  although  orthoclase  and  labradorite  are  present 
in  almost  equal  amounts,  the  latter  is  the  more  prominent  in  the  sections  and  is 
usually  given  more  weight  in  classification. 

BAGNOREAL  VICOSB.    II.  6.  2.  2. 

Megascopic  characters. — Medium  gray,  compact,  slightly  porphyritic.  Leucite  pheno- 
crysts few,  3  to  10  mm.,  grayish,  not  very  conspicuous.  Augite  phenocrysts  rare,  0.5  to  2  mm., 
prismatic,  black.  Groundmass:  medium  gray,  fine-grained,  phanerocrystalline. 

Microscopic  characters. — Holocrystalline,  magnophyric,  perpatic.  Phenocrysts:  10  per  cent 
or  less;  leucite,  augite.  Groundmass:  90  percent  or  more,  holocrystalline  or  percrystalline, 
intersertal  fabric,  leucite,  orthoclase,  labradorite,  augite,  magnetite,  apatite,  sometimes  nephelite 
or  glass. 

Orthoclase,  OrjAbi. — Phenocrysts:  none.  Groundmass:  about  18  per  cent,  in  part  o.  i  to 
0.5  mm.,  subhedral,  tabular,  with  diverse  arrangement,  in  part  as  anhedral,  formless  patches 
of  interstitial  cement. 

Labradorite,  AbjAn^. — Phenocrysts:  none.  Groundmass:  about  17  per  cent,  o.i  to  0.5 
mm.,  thin  tabular,  twinned,  arrangement  diverse. 

Leucite. — Phenocrysts:  about  5  per  cent,  3  to  10  mm.,  subhedral  to  anhedral,  equant  to 
irregular,  twinned,  inclusions  rare.  Groundmass:  about  30  per  cent,  o.  i  to  i  .o  mm.,  anhedral, 
equant  and  irregular,  few  inclusions. 

Nephelite. — Groundmass:   5  per  cent  to  none,  anhedral,  as  interstitial  cement. 

Augite.  —  Phenocrysts:  about  3  per  cent,  0.5  to  2.0  mm.,  subhedral  to  anhedral, 
prismatic  and  fragmentary,  pale  gray  or  greenish,  non-pleochroic,  inclusions  few.  Groundmass: 
about  15  per  cent,  o.i  to  0.5  mm.,  subhedral  to  anhedral,  prismatic  to  equant,  pale  gray  or 
very  pale  greenish. 

Magnetite. — Groundmass:  about  4  per  cent,  0.02  to  0.05  mm.,  anhedral,  equant  to 
irregular. 

Apatite. — Groundmass:    about  i  per  cent,  0.05  to  o.iomm.,  subhedral,  thin  prismatic. 

Glass. — Groundmass:  5  per  cent  to  none,  colorless,  difficult  to  detect. 


ioo  THE    ROMAN    COMAGMATIC     REGION. 

Chemical  composition  and  norm  as  on  p.  97. 

Type  specimens  from   Poggio  Cotognola   and   Madonna   del   Riposo,    near   Bracciano, 
Sabatinian  District. 

II.  6.  2.  2.    Orvietal  Vicose  [Leucite-Tephrite,  Orvieto  Type]. 

Megascopic  characters. — In  the  hand  specimen  these  rocks  are  quite  indistin- 
guishable from  the  orvietal  auruncose  described  above.  They  are  rather  light  gray 
basalts,  almost,  if  not  entirely,  aphyric,  the  only  phenocrysts  being  small  prisms  of 
dark-green  augite,  and  even  these  being  wanting  in  most  specimens,  and  when 
present  excessively  scarce.  The  grain  is  distinctly  phanerocrystalline,  but  very  fine,  a 
lens  being  needed  in  most  cases  to  distinguish  between  the  light  and  dark  minerals. 

Microscopic  characters. — In  thin  section  the  texture  is  seen  to  be  practically 
holocrystalline,  though  a  very  minute  amount  of  glass  may  be  sometimes  present. 
The  fabric  is  an  intersertal  one,  due  to  the  divergent  arrangement  of  the  labradorite 
laths.  This  labradorite,  which  gives  extinction  angles  corresponding  to  about 
AbxAnj,  is  in  thin  plates,  tabular  parallel  to  b  (oio),  and  almost  always  multiply 
twinned.  There  are  some  tables  of  alkali-feldspar  as  well,  but  these  are  few,  and 
for  the  most  part  this  mineral  is  anhedral  and  interstitial  between  the  other  com- 
ponents. Leucite  is  abundant,  rarely  in  subhedral  spheroids,  and  usually  in  irreg- 
ular anhedral  patches,  and  later  than  the  labradorite.  Its  double  refraction  is  rather 
weak,  and  inclusions  of  the  usual  kind  are  not  common  and  are  seldom  regularly 
arranged.  There  is  much  augite,  in  subhedral  or  anhedral  prismoids  and  grains,  of 
a  greenish-gray  color  and  non-pleochroic.  This  augite  is  often  accompanied  on  the 
ends  and  sides  by  a  later  growth  of  a  brown,  pleochroic  hornblende,  which  also 
forms  separate  anhedral  individuals.  The  amount  of  this  mineral,  which  may  be 
referred  to  barkevikite,  is  very  small.  A  little  olivine  is  also  seen,  in  small,  anhedral 
grains,  which  is  to  be  distinguished  from  the  augite  by  its  lack  of  color,  as  well  as 
by  its  higher  refractive  index  and  birefringence.  Grains  of  magnetite  and  small 
prisms  of  apatite  are  present  in  small  quantity.  There  is  also  a  very  small  amount 
of  interstitial  nephelite,  with  possibly  a  very  little  glass  in  some  specimens;  but  the 
rock  typically  is  holocrystalline. 

From  the  description  just  given  it  is  clear  that,  with  the  exception  of  the  presence 
of  barkevikite,  whose  amount  is  so  small  as  to  be  negligible  in  defining  the  type, 
this  rock  is  essentially  similar  to  the  groundmass  of  the  bagnoreal  type  described 
above.  It  may  be  regarded  as  the  aphyric  end  of  a  textural  series  of  types,  of  which 
the  viterbal  is  the  most  porphyritic,  and  the  bagnoreal  intermediate. 

Chemical  composition. — The  previously  published  analysis  of  this  type  is 
repeated  here,  with  a  number  of  additional  determinations  made  recently.  The 
present  statement  contains  a  correction  in  the  amount  of  Na2O,  as  it  was  found,  in 
going  over  the  figures  in  the  laboratory  notebook,  that  a  slip  had  been  made  in  sub- 
tracting the  weight  of  the  KC1  from  that  of  the  mixed  NaCl+KCl,  the  weight  of 
NaCl  being  given  as  0.05673  grm.,  instead  of  0.04673  grm.,  the  true  one. 


PETROGRAPHY. 

Chemical  Composition  of  Orvietal  Vicose  [Leucite-tephrite]. 


IOI 


I. 

I. 

SiO2  

CO    2A 

o  8*8 

TiO2  . 

A12O3  

I  *43 

.181 

ZrO2  

OI5 

Fe2O3  

2.  C4. 

.016 

P2Oe. 

FeO  

c.68 

•  O7O 

SO3  

MeO  .  . 

3.6=: 

.001 

s  

CaO  

7.8? 

.  I?Q 

MnO  

n  d 

Na2O  

*2.4.=\ 

.Od.O 

BaO  

K2O  

7.4.C 

080 

SrO  

H2O  + 

CO2  

none 

100.58 

*  Former  figure =2. 07. 

I.  Orvietal  vicose  [leucite-tephrite].     Monte  Cavallo,  near  Orvieto,  Vulsinian  District, 
ington,  analyst.     Jour.  Geol.,  V,  1897,  p.  370. 


Wash- 


Norm. 

Or 40.03 

16.96 


Ratios. 


An. 
Lc. 
Ne. 
Di. 
01. 
Mt. 
II.. 
Ap. 


56.99 


3-49{i4.85 
11.36)    •" 

15.46 
5-74 


5-99 
1.03 


71.84 


28.22 


Rest. 


Order  

Fern 
F 

-z-54 

—  2  ft  A 

Rang 

"L 

Subrane.  . 

CaO' 
K2O' 

1.97 
=  2.00 

2.28 

1-03 

100.06 

0.65 

100.71 

The  analysis  resembles  that  of  the  bagnoreal  type,  though  potash  is  here  some- 
what lower,  and  the  general  character  slightly  more  femic.  It  will  be  noted  that 
the  correction  of  the  figure  for  Na2O  not  only  lowers  the  previous  summation, 
which  would  be  101.03  with  the  additional  determination  of  BaO,  to  a  satisfactory 
figure,  but  also  carries  the  magmatic  position  from  sodipotassic  borolanose  (II.  6. 
2.  3),  where  the  type  was  formerly  reckoned  as  belonging,*  to  the  dopotassic  subrang 
vicose  (II.  6.  2.  2),  and  thus  in  accord  with  the  numerous  other  similar  leucitic  rocks 
of  the  region,  which  are  all  dopotassic. 

Mode. — As  was  the  case  with  the  groundmass  of  the  preceding  type,  this  rock 
did  not  lend  itself  well  to  measurement  of  the  mode  under  the  microscope,  though 
a  rough  estimate  was  made  to  control  the  calculation  of  the  mode  from  the  norm, 
which  was  carried  out  as  usual.  In  estimating  the  barkevikite  this  mineral  was 
assumed  to  have  the  composition  of  the  similar  hornblende  in  the  pulaskose  (soda- 
lite-syenite)  of  Square  Butte.f 

*  Washington.  Prof.  Paper  U.  S.  Geol.  Surv.  No.  14,  1003,  p.  297.  It  was  remarked  here  that  it  is  "near 
subrang  2  of  essexase,"  that  is,  vicose. 

t  Lindgren  and  Melville,  Am.  Jour.  Sci.,  XLV,  1893,  p.  202.  Cf.  Pirsson,  Bull.  U.  S.  Geol.  Surv.  No.  237, 
i cos,  p.  66. 


IO2 


THE  ROMAN  COMAGMATIC  REGION. 


Orthoclase,   Or3 Abr 10.5 

Labradorite,  OrjAb! 26.  i 

Leucite 29 .  o 

Nephelite  .....'. 2.8 

Augite 21.6 

Barkevikite 2.0 


Olivine 5.0 

Magnetite 2.0 

Apatite i .  o 


In  this  mode  the  decidedly  alferric  character  and  the  predominance  of  labra- 
dorite  over  orthoclase  are  marked.  As  compared  with  the  viterbal  and  bagnoreal 
types  of  vicose  the  alkali-feldspar  contains  a  much  smaller  proportion  of  the  albite 
molecule,  while  the  labradorite  is  distinctly  more  sodic,  being  on  the  border  of 
andesine.  In  these  respects  it  corresponds  very  closely  with  the  mode  of  orvietal 
auruncose  described  above. 

As  compared  with  the  norm  it  is  clear  that  the  only  differences  of  serious 
importance  are,  as  usual,  those  involved  in  the  formation  of  leucite,  much  of  the 
normative  orthoclase  being  used  for  this,  and  the  nephelite,  changed  to  albite, 
entering  into  the  feldspars.  The  type  may  therefore  be  described  as  leucite  interserti- 
aphyro-vicose. 

Occurrence. — So  far  as  known  orvietal  vicose  occurs  only  in  the  Vulsinian  Dis- 
trict. It  is  abundant  near  the  town  of  Orvieto,  especially  to  the  south  of  this,  as 
at  Monte  Cavallo  and  Porano,  where  it  forms  extensive  flows.  It  also  makes  up 
the  main  part  of  the  hill  of  Monte  Bisenzo  on  the  southwest  shore  of  Lake  Bolsena. 

Name. — The  derivation  of  the  subrang  name  is  discussed  above.  That  of 
the  type  is  based  on  the  name  of  the  town  of  Orvieto,  near  which  it  occurs  in 
abundance. 

In  the  prevailing  systems  of  classification  the  name  of  leucite-tephrite,  which 
was  applied  to  the  type  in  a  previous  description,  is  the  appropriate  one,  on  account 
of  the  quantitative  predominance  of  labradorite  over  orthoclase,  as  well  as  its  greater 
conspicuousness  in  the  thin  section.  The  amount  of  olivine  is  so  small  that  its 
presence  may  be  disregarded,  and  the  name  of  leucite-basanite  is  unwarranted. 

ORVIETAL  VICOSE.    II.  6.  2.  2. 

Megascopic  characters. — Medium  gray,  compact,  aphyric,  very  fine-grained,  phanero- 
crystalline. 

Microscopic  characters. — Holocrystalline,  microporphyritic,  intersertal  fabric.  Micropheno- 
crysts:  about  50  per  cent,  leucite,  labradorite,  orthoclase,  augite.  Microgroundmass:  about  50 
per  cent,  augite,  orthoclase,  olivine,  barkevikite,  nephelite,  magnetite,  apatite. 

Orthoclase,  Or3AbI. — Microphenocrysts:  about  3  per  cent,  o.i  to  0.5  mm.,  subhedral, 
thin  tabular,  twinning  uncommon,  arrangement  diverse.  Microgroundmass :  about  8  per  cent, 
anhedral,  as  interstitial  areas. 

Labradorite,  AbjAnj. — Microphenocrysts:  about  25  percent,  o.i  to  0.5  mm.,  subhedral 
thin,  tabular,  twinning  common,  arrangement  diverse. 

Leucite. — Microphenocrysts:  about  30  per  cent,  o.i  to  i.omm.,  anhedral,  equant,  more 
often  irregular,  often  incloses  the  feldspar  tables,  inclusions  few. 

Nephelite. — Microgroundmass:  about  2  per  cent,  anhedral,  formless,  interstitial  areas. 
Not  always  present. 


PETROGRAPHY.  103 

Augite. — Microphenocrysts  and  microgroundmass:  about  20  per  cent,  o.i  to  0.5  mm., 
subhedral  to  anhedral,  stout  prismatic  and  equant,  pale  greenish-gray,  non-pleochroic. 

Olivine. — Microgroundmass:  about  5  per  cent,  0.08  to  o.  10  mm.,  subhedral  to  anhedral, 
equant,  colorless. 

Barkevikite. — Microgroundmass;  about  2  per  cent,  0.02  to  0.05  mm.,  anhedral,  as  inter- 
stitial areas  or  fringing  the  augites,  light  brown. 

Magnetite. — Microgroundmass:   about  2  per  cent,  0.02  to  0.05  mm.,  anhedral,  equant. 

Apatite. — Microgroundmass:    about  i  per  cent,  subhedral,  prismatic. 

Chemical  composition  and  norm  as  on  p.  101. 

Type  specimen  from  Monte  Cavallo,  south  of  Orvieto,  Vulsinian  District. 

II.  7.  2.  2.    Vesbal  Braccianose  [Leucite-Tephrite,  Vesuvius  Type] . 

Megascopic  characters. — Rocks  of  this  type  are  of  a  general  dark-gray  color, 
speckled  with  numerous  small  white  spots.  They  are  highly  porphyritic,  the  very 
great  majority  of  the  phenocrysts  being  of  leucite,  and  with  small  and  almost  negli- 
gible numbers  of  phenocrysts  of  augite  and  sometimes  of  olivine  or  biotite.  The 
leucite  phenocrysts  vary  in  diameter  from  i  to  3  mm.,  being  seldom  over  the  latter. 
They  are  for  the  most  part  spheroids,  giving  round  sections,  but  are  also  well-formed 
trapezohedra.  In  color  they  are  usually  pale  gray,  rather  than  clear  white,  so  that 
they  are  not  as  strikingly  conspicuous  as  might  be  expected,  though  always  clearly 
discernible.  The  few  augite  phenocrysts  are  dark  green,  in  small,  stout  prismoids 
and  grains,  seldom  over  3  mm.  long.  The  more  uncommon  olivine  phenocrysts 
are  of  similar-sized,  yellowish-green  grains,  and  the  still  rarer  biotites  form  small 
black  tables.  The  groundmass  is  a  dark  gray,  compact  and  aphanitic,  and  in  the 
field  these  rocks  would  be  called  leucite  melaphyres.  Most  of  the  specimens  are 
more  or  less  vesicular. 

Microscopic  characters. — In  thin  section  the  leucite  phenocrysts,  which  form 
nearly  one-half  of  the  rock  volume,  present  mostly  rounded  outlines,  with  occasional 
crystal  planes.  They  are  apt  to  be  cracked,  or  sometimes  in  clusters  of  several  closely 
juxtaposed  individuals.  Double  refraction  is  not  well  marked,  as  a  rule,  and  inclu- 
sions are  generally  few  and  irregularly  arranged.  An  occasional  stout  and  pris- 
matic labradorite  phenocryst  is  seen,  though  these  are  seldom  if  ever  visible  in  the 
hand  specimen.  The  augite  phenocrysts  are  in  stout  prismoids,  usually  much 
broken  and  often  anhedral,  of  a  slightly  yellowish-brown  tinge  of  gray.  They 
carry  few  inclusions,  and  these  mostly  of  magnetite.  The  rare,  anhedral,  generally 
fragmentary  olivine  phenocrysts  call  for  no  comment,  nor  do  the  still  rarer  tables  of 
brownish  biotite,  which  show  the  common  alteration  phenomena. 

The  groundmass  in  which  these  lie  is  usually  holocrystalline,  but  a  trifling 
amount  of  glass  is  visible  in  some  instances.  The  fabric  is  xenomorphic  granular, 
somewhat  intersertal  through  the  development  and  diverse  arrangement  of  the  labra- 
dorite, and  may  be  briefly  described  as  subintersertal.  It  consists  in  large  part  of 
pale-gray,  slightly  brownish  augite,  sometimes  showing  a  zonal  structure,  in  irreg- 
ular anhedral  grains,  few  of  which  assume  a  distinctly  prismatic  habit.  Tables  of 
labradorite,  which  give  rise  to  lath-shaped  sections,  are  fairly  numerous,  their  length 


IO4 


THE  ROMAN   COMAGMATIC  REGION. 


often  reaching  i  mm.,  but  usually  much  under  this.  They  are  always  multiply 
twinned,  with  extinction  angles  which  vary  somewhat,  but  which  indicate  an  average 
composition  of  about  Ab2An3.  As  stated  above,  their  arrangement  is  divergent, 
so  as  to  give  rise  to  an  approach  to  a  true  intersertal  fabric.  Alkali-feldspar  seems 
to  be  quite  absent,  but  some  small  leucite  anhedra  are  found  in  the  groundmass. 
Small  olivine  grains,  usually  quite  anhedral,  but  occasionally  with  crystal  planes, 
are  scattered  through  the  groundmass,  but  the  total  amount  of  this  mineral  is  always 
small  and  quite  negligible  for  classificatory  purposes.  A  little  residual  nephelite 
base  is  present  in  most  cases,  its  place  being  taken  in  some  specimens  by  glass. 

Chemical  composition. — An  analysis  of  a  specimen  of  this  type  from  the  flow  of 
1872  of  Mount  Vesuvius,  collected  below  the  Observatory,  is  given  below  in  I,  hav- 
ing been  previously  published,  but  without  petrographical  description.  The  posi- 
tion assigned  the  rock  at  that  time,  namely,  in  vesuvose,  II.  8.  2.  2,  was  erroneous 
through  inadvertence,  not  on  account  of  error  in  the  analysis  or  in  calculating  the 
norm.  As  was  pointed  out  later,*  the  analysis  should  have  been  placed  in  the 
subrang  II.  7.  2.  2.  With  it  is  also  given  an  analysis  of  the  lava  of  the  eruption  of 
1891-93,  which  probably  belongs  to  the  same  type,  though  the  description  of  Mat- 
teuccif  leaves  this  somewhat  in  doubt. 

Chemical  Composition  of  Vesbal  Braccianose  \Leucite-tephrite\. 


I. 

II. 

I. 

II. 

SiO2  

4.7.6?       O    1QA- 

4.8.  OQ 

CO2  

none 

A12O3  

18  i?         i  78 

19.82 

TiO2  

I     13       O    OI4. 

Fe2O3  

2    67              OI7 

2     CO 

ZrO2 

o  02 

FeO  

6.48           .OOO 

5  .  26 

P2O,  .  . 

o.  ?o       .  004. 

O.  33 

MgO  

4.  IQ           .  ICX 

2.82 

SO,  .  . 

trace 

CaO  

o.oi       .  161 

8.n 

MnO  

nd. 

Na2O  

2.78        .04? 

3.17 

BaO  

O.  24. 

Ko 

H2O  +  

O.  I?  ) 

IOO-47 

IOO.I7 

H2O  

6  \ 

O.  II   \ 

n.d. 

I.  Vesbal  braccianose  [leucite-tephrite].     Lava  of  1872,  below  Observatory,  Mount  Vesuvius. 

Cf.  Washington,  Prof.  Paper  U.  S.  G.  S.  No.  14,  1903,  p.  307. 

II.  Vesbal  ( ?)  braccianose  [leucite-tephrite].      Lava  of  1891-93.      Atrio  del  Cavallo,  Mount 

Vesuvius.     Mrha,  analyst.     F.  Becke,  Tsch.  Min.  Pet.  Mitth.,  XVIII,  1898,  p.  94. 


•1.88 


=  0.80 


=  1.78 


Or  

Non 

14 
14 
23 

12 
21 

5 
3 

2 

I 

i  of  I. 
46  ) 

73  r9-19 
^{36.32 

fy- 

?J}  m 

3°       i-3° 

)                    Class  

Ratios  of  I. 
Sal 

An 

Lc  

)    5'SI 
Order  

Fern 
F 

Ne  

Di 

Ol  

L 
K2O'+Na2O' 

Mt  

11  

Ap  .. 

Subrang  

CaO' 
K2O' 

Rest 

IOO 
O 

27 

5° 

*  Washington,  Prof, 
t  R.  V.  Matteucci. 

"Na2O' 

100.77 

Paper  U.  S.  Geol.  Surv.  No.  28,  1904,  p.  68. 
Tscherm.  Min.  Pet.  Mitth.,  XV,  1895,  p.  342. 

PETROGRAPHY.  105 

The  analysis  (I)  resembles  closely  many  of  the  earlier  analyses  of  Vesuvian  lavas. 
Compared  with  the  analyses  given  in  the  preceding  pages  it  is  marked  by  its  low 
silica  content,  which  is  3  per  cent  less  than  in  the  types  of  vicose.  Concomitantly 
with  this  decrease  in  silica  there  is  an  increase  in  the  amount  of  lime,  though  the 
other  constituents  are  not  notably  affected.  The  analysis  of  the  later  flow 
resembles  the  former  in  a  general  way,  though  potash  is  decidedly  higher.  The 
higher  alumina  is  to  be  attributed  in  part  to  the  non-determination  of  TiO2, 
which  would  be  included  in  this,  and  also  possibly  in  part  to  incomplete  separation 
of  the  magnesia,  as  the  amount  of  this  last  is  rather  low. 

Mode. — In  case  of  this  type,  as  in  some  of  the  preceding,  the  character  of  the 
groundmass  did  not  permit  of  satisfactory  measurement  by  Rosiwal's  method,  the 
relative  amount  of  leucite  phenocrysts  being  the  only  quantitative  relation  which 
could  be  determined  with  precision.  The  mode  was  calculated  from  the  norm 
on  the  assumptions  that  no  orthoclase  was  present  and  that  all  the  potash  belongs 
to  leucite,  that  the  normative  olivine  corresponds  with  the  modal,  that  the  augite 
has  the  composition  of  that  of  Ticchiena  (p.  134),  and  that  the  plagioclase  has  the 
composition  Ab2An3.  The  results  of  this  calculation  are: 

Labradorite,  Ab2An3 *7-4 

Leucite 34-9 

Nephelite 9.0 

Augite 30. 3 

Olivine 5.6 

Ores 1.5 

Apatite 1.3 


The  amount  of  leucite  shown  corresponds  well  with  that  determined  optically, 
namely,  40  per  cent  by  volume,  and  in  general  the  mode  above  agrees  with  the 
appearance  of  the  thin  section.  Only  the  amount  of  nephelite  seems  to  be  high.  The 
figure  for  this  was  that  obtained  by  the  usual  equations  involving  the  total  soda  and 
the  silica  available  after  formation  of  leucite,  augite,  olivine,  and  the  anorthite  of 
plagioclase.  These  give  an  amount  of  albite  molecule  exactly  right  to  satisfy  the 
composition  of  the  plagioclase  determined  optically,  that  is,  Ab  =0.013,  An=o.o38. 
Any  notable  change  in  the  figure  for  nephelite  will  involve  very  serious  modification 
of  the  plagioclase  composition,  rendering  it  an  andesine,  which  can  not  be  considered 
in  view  of  the  optical  determinations,  and  will  also  concomitantly  lead  to  increase 
in  the  amount  of  olivine  and  decrease  in  that  of  augite,  which  are  also  inadmissible. 
The  amount  of  nephelite  is  readily  underestimated  by  the  eye  in  such  groundmasses, 
and  a  test  of  the  rock  powder  with  very  dilute  nitric  acid,  according  to  the  method 
suggested  by  Pirsson,*  gave  abundant  gelatinous  silica,  thus  proving  its  presence 
in  considerable  amount. 

Occurrence. — This  type  is  abundant  in  the  Campanian  District,  many  of  the 
flows  of  Vesuvius  belonging  to  it.  Among  those  which  I  have  examined  may  be 
mentioned  those  of  1822  (above  Cerasiello)  and  1872  (below  the  Observatory), 

*  L.  V.  Pirsson,  Am.  Jour.  Sci.,  II,   1896,  p.  142. 


io6 


THE  ROMAN  COMAGMATIC  REGION. 


while  the  descriptions  of  Fuchs  suggest  that  flows  of  1731,  1754,  1767,  1779,  1786, 
1802,  1810,  1855,  1858,  and  1867  are  also  of  this  type.  The  type  seems  also  to 
occur  at  Monte  Somma,  but  to  a  more  limited  extent.  Outside  of  the  Campanian 
District  these  rocks  seem  to  be  rare,  and  the  only  specimen  in  my  possession  which 
may  probably  be  referred  here  is  the  lava  of  Poggio  Romolo,  south  of  Lake  Bolsena. 

Name. — The  name  of  the  subrang  is  derived  from  the  lake  and  town  of  Brac- 
ciano,  the  chief  geographical  features  of  the  Sabatinian  District,  in  the  southern 
part  of  which  this  magma  is  very  abundant.  The  type  name  is  derived  from 
Vesbius,  an  old  Latin  name  of  Vesuvius. 

In  the  prevailing  classifications  the  type  should  be  regarded  as  a  leucite-teph- 
rite,  since  the  amount  of  olivine  is  so  small  that  the  name  of  leucite-basanite  often 
given  it  is  not  justified  if  regard  is  paid  to  quantitative  relations. 

VBSBAL  BRACCIANOSE.    II.  7.  2.  2. 

Megascopic  characters. — Dark  gray,  sprinkled  with  small  white  spots.  Compact  to  vesic- 
ular, highly  porphyritic.  Leucite  phenocrysts  very  abundant,  i  to  3  mm.,  round,  white  or  pale 
gray,  sometimes  not  very  conspicuous.  Augite  phenocrysts  very  few,  i  to  3  mm.  long,  prismatic 
black  or  dark  green.  Olivine  phenocrysts  very  rare,  not  essential,  i  to  2  mm.,  equant,  yellow. 
Biotite  phenocrysts  very  rare,  not  essential,  i  to  3  mm.,  tabular,  brown.  Groundmass,  rather 
dark  gray,  aphanitic. 

Microscopic  characters. — Holocrystalline,  mediophyric,  sempatic.  Phenocrysts:  about  40 
per  cent,  leucite,  augite,  sometimes  olivine  and  biotite.  Groundmass:  about  60  per  cent,  holo- 
crystalline,  somewhat  intersertal  fabric,  augite,  labradorite,  nephelite,  leucite,  olivine,  magnetite, 
apatite. 

Labradorite,  Ab2An3. — Phenocrysts:  2  to  o  per  cent,  i  to  2  mm.,  subhedral,  stout  prismatic, 
twinned,  few  inclusions.  Groundmass:  about  15  per  cent,  0.05  to  0.50  mm.,  thin  prismatic 
or  tabular,  much  twinned,  arrangement  diverse. 

Leucite. — Phenocrysts:  about  35  percent,  0.5  to  3.0  mm.,  subhedral  to  anhedral,  equant  to 
irregular,  sometimes  in  clusters,  inclusions  few.  Groundmass :  about  2  per  cent,  o .  05  to  o .  10  mm., 
anhedral,  equant  and  irregular. 

Nephelite. — Groundmass:   5  to  10  per  cent,  anhedral,  as  interstitial  cement. 

Augite. — Phenocrysts:  about  5  per  cent,  often  less,  0.5  to  3.0  mm.,  subhedral  to  anhedral, 
prismatic  or  fragmentary,  pale  yellowish-brown  or  yellowish-green,  uonpleochroic,  inclusions 
few.  Groundmass:  about  25  per  cent,  0.05  to  0.20  mm.,  anhedral,  prismatic,  fusiform, 
equant,  pale  greenish-  or  yellowish-gray,  arrangement  generally  diverse,  sometimes  tangential 
about  the  leucite  phenocrysts,  no  inclusions. 

Biotite. — Phenocrysts:  2  to  o  per  cent,  0.5  to  3.0  mm.,  subhedral,  tabular,  brown,  usually 
altered. 

Olivine. — Phenocrysts:  2  to  o  per  cent,  0.5  to  2.0  mm.,  subhedral  to  anhedral,  equant 
to  irregular,  colorless.  Groundmass:  about  3  per  cent,  not  always  present,  0.05  to  o.iomm., 
anhedral,  equant,  colorless. 

Magnetite. — Groundmass:  about  2  per  cent,  0.02  to  0.05  mm.,  anhedral,  equant,  and 
irregular. 

Apatite. — Groundmass:    about  i  per  cent,  0.05  to  0.20  mm.,  subhedral,  prismatic. 

Chemical  composition  and  norm  as  on  p.  104. 

Type  specimen  from  flow  of  1872,  below  Observatory,  Mount  Vesuvius,  Campanian 
District. 


PETROGRAPHY.  107 

II.  7.  2.  2.  Sommal  Braccianose  [Leucite-Tephrite,  Sotntna  Type]. 

Megascopic  characters. — Like  the  preceding  type,  this  is  highly  porphyritic. 
The  phenocrysts  are  of  both  leucite  and  augite,  present  in  about  equal  amount  and 
together  making  up  about  40  per  cent  or  so  of  the  rock  volume.  Those  of  leucite 
are  from  i  to  4  mm.  in  diameter,  rounded,  and  anhedral,  with  only  occasional  crystal 
planes.  Their  color  is  usually  gray,  so  that  they  do  not  stand  out  prominently 
against  the  groundmass.  The  phenocrysts  of  augite  are  about  the  same  size,  usually 
subhedral  and  with  crystal  planes,  especially  those  of  the  prisms  and  pinacoids. 
In  form  they  tend  to  be  equant  rather  than  prismatic,  giving  square  or  only  slightly 
oblong  and  angular  sections.  They  are  very  dark  green  in  color,  almost  black.  An 
occasional  yellowish-green  grain  of  olivine  is  seen,  but  the  amount  of  this  mineral 
is  negligible.  The  groundmass  is  dark  gray  and  aphanitic.  In  the  field  the  type 
would  be  called  a  leucite-augite-melaphyre. 

Microscopic  characters. — The  thin  sections  of  this  type  present  much  the  same 
appearance  as  those  of  the  vesbal.  There  are  the  same  rounded  leucite  phenocrysts, 
showing  feeble  double  refraction  and  carrying  few  inclusions,  but  they  are  not 
nearly  as  thickly  scattered  as  in  the  preceding  type.  On  the  other  hand,  pheno- 
crysts of  pale-gray  augite  are  much  more  abundant,  and  one  or  two  subhedral  olivine 
phenocrysts  are  seen  in  the  section. 

The  groundmass  between  these  is  like  the  preceding,  especially  in  the  plagio- 
clase  laths  and  their  diverse  arrangement,  giving  rise  to  a  somewhat  intersertal  fabric. 
Leucite  anhedra  are  common,  this  mineral  often  forming  irregular  interstitial  areas. 
Small  grains  of  augite  are  abundant,  though  less  so  than  in  the  preceding  type,  as 
much  of  this  mineral  is  here  developed  phenocrystically.  There  are  also  small 
anhedra  of  olivine  and  considerable  residual  base,  which  may  be  of  glass,  but  is 
nephelite  for  the  most  part.  The  usual  magnetite  grains  and  apatite  prisms  may 
also  be  mentioned. 

Chemical  composition  and  mode. — No'analysis  has  been  made  of  a  rock  of  this 
type,  so  that  it  is  uncertain  whether  the  classificatory  position  assigned  to  it  is  correct 
or  not.  As,  however,  all  the  other  Vesuvian  lavas  analyzed  by  me,  and  many  of 
those  analyzed  by  others,  fall  in  braccianose,  there  is  little  room  for  doubt  that 
some,  if  not  most,  of  the  rocks  with  this  mode  and  texture  are  really  in  this  subrang. 

The  mode  could  not  be  satisfactorily  estimated  by  microscopic  methods,  but 
the  study  of  the  thin  sections  indicates  clearly  that  it  must  be  quite  similar  to  that 
of  the  vesbal  type,  though  possibly  with  slightly  greater  relative  amounts  of  alferric 
minerals. 

Occurrence. — The  most  typical  occurrence  of  this  rock  is  the  flow  of  1754  near 
Bosco  Reale  at  Mount  Vesuvius.  Other  flows  which  belong  to  this  type,  as  deter- 
mined by  specimens  examined  by  me,  are  those  of  17 50, 1751,  1771,  1834,  and  1858, 
while  the  descriptions  of  Fuchs  lead  one  to  refer  here  as  well  those  of  1809,  1832, 
and  1839.  The  type  is  also  abundant  at  the  encircling  Monte  Somma,  both  in  the 
lava  flows  and  the  dikes. 


io8  THE  ROMAN  COMAGMATIC  REGION. 

Name. — The  type  derives  its  name  from  Monte  Somma,  where  it  occurs  abun- 
dantly. In  the  prevailing  systems  of  classification  it  would  be  called  a  leucite- 
tephrite,  the  small  amount  of  olivine  being  regarded  as  negligible. 

II.  7.  2.  2.    Galeral  Braccianose  [Leucitite,  (ialera  Type]. 

Field  characters. — This  type  is  very  dark,  compact  and  heavy.  It  is  only 
slightly  porphyritic  megascopically  or  aphyric,  phenocrysts  of  leucite  making  up 
less  than  5  per  cent  of  the  rock  volume  and  sometimes  being  absent.  They  are 
usually  well-formed  trapezohedra,  often  fragmentary,  white  or  very  pale  gray,  and 
often  with  a  slightly  waxy  luster,  and  vary  in  diameter  from  5  to  10  mm.  Small 
phenocrysts  of  augite  are  either  wholly  wanting  (the  typical  case)  or  are  present  in 
extremely  small  quantities.  No  other  phenocrysts  are  found.  The  groundmass  is 
a  typical  basaltic  one,  very  dark  gray  or  almost  black,  fine-grained,  and  aphanitic, 
but  without  a  vitreous  luster.  Some  of  the  specimens  show  a  few  vesicles,  but  the 
majority  are  quite  free  from  them.  The  type  would  be  called  a  leucitic  melaphyre 
or  a  basalt  in  the  field. 

Microscopic  characters. — The  very  few  large  leucites  seen  in  the  thin  sectior 
show  the  usual  characters,  good  development  of  the  crystalline  form,  often  fragmen- 
tary, pronounced  birefringence,  and  comparatively  few  inclusions.  The  still  rarer 
large  augite  phenocrysts  call  for  no  remark,  as  they  are  of  the  type  common  in  the 
region,  and  already  often  described. 

The  groundmass  is  percrystalline  and  microporphyritic,  and  shows  a  fabric 
which  is  highly  characteristic  of  many  of  the  more  femic  leucite  rocks  of  the  region. 
Leucite  microphenocrysts  are  abundant,  making  up  about  one-third  of  the  rock. 
They  present  rounded  circular  sections,  and  are  usually  anhedral,  though  occa- 
sional crystal  planes  are  seen.  Their  size  is  fairly  uniform,  mostly  between  0.20 
and  0.30  mm.  in  diameter,  though  smaller  and  slightly  larger  individuals  occur. 
Double  refraction  is  faint,  but  readily  seen  with  the  selenite  plate.  Inclusions  are 
apt  to  be  few,  but  some  specimens  show  many  small  ones,  of  glass,  augite,  or  mag- 
netite, which  usually  form  a  circular  ring  near  the  periphery  of  the  crystal,  or  else 
are  arranged  so  as  to  show  a  skeletal  development  of  the  leucite.  Rarely  the 
original  skeleton  forms  are  well  preserved. 

Between  these  leucites  is  the  microgroundmass,  which  consists  in  great  part  of 
a  felted  mass  of  very  minute,  pale,  greenish-gray  prisms  of  augite,  with  occasional 
larger  individuals,  which  are  to  be  regarded  as  microphenocrysts.  While  the  gen- 
eral arrangement  of  these  small  prisms  is  diverse,  adjacent  to  the  leucites  they 
are  tangential,  as  if  they  had  been  present  in  the  still  molten  magma  prior  to  the 
complete  crystallization  of  the  salic  mineral  and  had  been  shoved  aside  during  its 
growth.  With  these  augiteS  are  small  grains  of  magnetite,  as  well  as  some  laths  of 
feldspar,  which  is  mostly  a  labradorite. 

Acting  as  a  cement  for  these  crystals  is  a  clear,  colorless  substance,  the  amount 
of  which  is  very  difficult  to  estimate  on  account  of  the  close  juxtaposition  and  mostly 


PETROGRAPHY. 


109 


felted  arrangement  of  the  numerous  augite  prismoids.  In  part  this  is  seen  between 
crossed  nicols  to  be  feldspar — both  labradorite  and  to  a  less  extent  orthoclase — in 
micropoikilitic  patches,  and  in  part  an  isotropic  glass  of  about  the  same  refractive 
index  as  the  leucite,  though  usually  slightly  lower. 

In  a  general  way  this  groundmass  resembles  in  mode  and  fabric  the  vesbal 
type,  though  on  a  much  smaller  scale  and  with  a  much  more  pronouncedly  tangen- 
tial arrangement  of  the  augite  prismoids.  The  effect,  especially  when  the  leucites 
carry  few  or  no  inclusions,  is  that  of  a  net  or  section  of  a  sponge,  the  felted  mass  of 
augite  prismoids  representing  the  threads  or  walls  and  the  clear,  colorless,  round 
leucites  the  holes.  This  microtexture  is  so  characteristic  of  many  of  the  rocks  of 
the  region,  and  so  easily  recognizable  that  it  is  well-deserving  of  a  special  name. 
For  this  the  term  clathrate*  may  be  used,  borrowed  from  the  glossaries  of  botany 
and  zoology.  It  is,  of  course,  conceivable  that  a  similar  textural  habit  may  be  pro- 
duced by  the  equant,  spheroidal  development  of  other  salic  minerals,  as  quartz, 
feldspar,  nephelite,  or  the  sodalites,  along  with  the  similar  interstitial  felt  of 
augite  or  other  alferric  prismoids,  though  this  is  obviously  not  a  very  probable 
occurrence,  and  is  as  yet  unknown  as  far  as  my  knowledge  extends,  the  texture 
being  confined  to  leucite-augite  rocks. 

As  this  habit,  characterized  by  the  development  of  leucite  in  round  micropheno- 
crysts,  surrounded  by  smaller  and  more  or  less  tangential  subhedral  to  anhedral 
augite  prisms,  is  very  common  in  the  Roman  Region  and  characteristic  of  many  of 
the  more  f  emic  rocks,  it  may  well  be  given  a  special  name.  For  this  the  term  galeroid 
would  seem  appropriate. 

Chemical  composition. — An  analysis  of  this  type,  published  in  incomplete 
form  some  years  ago,  is  given  below,  with  the  addition  of  determinations  of  some 
of  the  minor  constituents.  The  analysis  of  vesbal  braccianose  is  also  repeated  for 
comparison. 

Chemical  Composition  of  Galeral  Braccianose  [Leucitite]. 


I. 

11. 

I. 

II. 

SiO2  

47.80     0.708 

47.6?     0.704 

CO2  

none 

none 

A12O3  

17.87         .17? 

18.1*       .178 

TiO2  

o.  77     o.oio 

i  .  13     0.014 

Fe,O,  . 

4.Q?             .  O3I 

2.6^          OI7 

ZrO2  

O.O2 

O.O2 

FeO         .... 

•?   64         .050 

6  .  48       .  090 

P2O5  .  . 

o.  36       .003 

O.  ?O          .OO4 

MgO    

3.68       .002 

4.  10         .lO? 

SO3  

0.06 

trace 

CaO  

8.  70       .  i?? 

o.  01       .  161 

MnO  

n.d. 

n.<L 

Na2O  

2  .  60       .  042 

2.78       .04? 

BaO  

0.28 

0.24 

Kn 

80-?                    I~>8T 

H20+   

0.65 

0.13 

99.68 

100.47 

H2O-   

O.II 

Sp-  gr  

2.  70! 

I  .  Galeral  braccianose  [leucitite].     West  of  Crocicchie,  south  of  Lake   Bracciano,  Sabatinian 

District.     Washington,  analyst.     Jour.  Geol.,  V,  1897,  p.  49. 
II.  Vesbal  braccianose  [leucite-tephrite].     Lava  of  1872,  below  Observatory,  Mount  Vesuvius. 

Cf.  p.  104. 
*  From  Latin  clathratus,  latticed. 


no  THE  ROMAN  COMAGMATIC  REGION. 

Norm  of  I.  Ratios  of  I. 


Or  

20  8? 

An  

7Q 

,  21.  58 

Ne  

11  

21  .  67 

Ol  

Mt  

.     7.  10 

Ap.. 

.     O.  Q2 

Rest  

98.59 
,  I  .  OI 

Sal 


Order  

F 

"L 

Subrane  .  . 

K2O' 

0.92 

' 

99.60 

The  analysis  shows  the  generally  central  position  of  this  type  in  the  various 
classificatory  divisions,  in  which  it  differs  somewhat  from  the  other,  though  the 
similarity  between  the  two  analyses  is  marked. 

Mode. — Owing  to  the  character  of  the  groundmass,  Rosiwal's  method  is  not 
applicable  to  this  type,  though  the  relative  amount  by  volume  of  the  leucite  micro- 
phenocrysts  was  determined  rather  satisfactorily.  Recalculation  of  the  norm  was 
somewhat  uncertain,  because  of  ignorance  of  the  relative  amounts  of  alkali-feldspar 
and  nephelite,  but  as  the  amount  of  the  latter  was  small,  the  error  involved  in  the 
arbitrary  assumptions  made  as  to  it  will  not  affect  the  result  seriously.  Assuming 
that  it  has  the  composition  below  and  that  about  5  per  cent  is  present,  the  follow- 
ing mode  was  obtained,  which  may  be  regarded  as  closely  approximating  to  the 
truth.  The  amount  of  leucite  shown  agrees  reasonably  well  with  the  measurec 
amount,  especially  when  the  leucite  megaphenocrysts  are  taken  into  consideration, 
though  it  is  somewhat  higher.  The  nephelite  given  here  exists  for  the  most  part  as 

glass. 

Orthoclase,  Or,  Abr 5.5 

Labradorite,  Ab2 An3 13.9 

Leucite 36. 2 

Nephelite 8.3 

Augite 30. 4 

Ores 4.7 

Apatite i .  o 

100.0 

Occurrence. — This  type  is  especially  well  represented  in  the  Sabatinian  Dis- 
trict, practically  all  of  the  extensive  flows  of  leucite-melaphyre  (leucitite)  south  of 
Lake  Bracciano  belonging  to  it.  Some  prominent  localities  are  Crocicchie,  the  quarry 
of  L'Uomo  Morto,  Grotta  Lobbra,  La  Toraccia,  and  other  points  along  and  close  to 
the  south  shore,  and  the  Osteria  Nuova  and  elsewhere  in  the  neighborhood  of  Santa 
Maria  di  Galera.  The  type  also  occurs  in  the  Vulsinian  District,  as  at  Trebia- 
nello  and  II  Giglio  to  the  east  of  Lake  Bolsena,  and  probably  also  in  the  Latian 
and  Auruncan  districts,  though  none  of  my  specimens  can  be  definitely  referred 
to  this  subrang. 

Name. — The  type  name  is  derived  from  the  locality  of  Santa  Maria  di  Galera, 
one  of  the  oldest-known  occurrences. 


PETROGRAPHY.  in 

The  type  has  been  uniformly  called  leucitite  by  petrographers,  and  the  name  is 
justifiable  in  the  prevalent  systems,  on  acccount  of  the  small  amount  and  inconspicu- 
ousness  of  the  feldspars,  though  the  presence  of  these  in  the  Galera  rock,  and  its 
consequent  relation  to  the  leucite-tephrites,  is  noted  by  Rosenbusch. 

OALERAL  BRACCIANOSB.    II.  7.  2.  2. 

Megascopic  characters. — Very  dark  gray  or  black,  compact,  aphyric,  or  very  slightly 
porphyritic.  Leucite  phenocrysts  rare,  i  to  5  mm.,  euhedral,  equant,  white,  quite  conspicuous. 
Augite  phenocrysts  very  rare,  i  to  3  mm.,  prismatic,  black.  Groundmass,  very  dark  gray  or 
black,  aphanitic,  not  vitreous. 

Microscopic  characters. — Holocrystalline  to  percrystalline,  microporphyritic.  Megapheno- 
crysts,  none  to  5  per  cent,  leucite,  augite.  Microphenocrysts,  about  50  per  cent,  leucite,  augite. 
Microgroundmass,  about  50  per  cent,  augite,  labradorite,  nephelite  (not  always  present),  ortho- 
clase,  magnetite,  apatite,  glass  (not  always  present).  Groundmass  has  clathrate  fabric. 

Orthoclase,  OrtAbi. — Groundmass:  5  to  o  per  cent,  anhedral,  interstitial,  distinguishable 
with  difficulty. 

Labradorite,  Ab2An3. — Groundmass:  about  10  per  cent,  anhedral,  to  a  small  part  tabular, 
mostly  anhedral,  interstitial  patches. 

Leucite. — Megaphenocrysts:  2  to  o  per  cent,  i  to  5  mm.,  euhedral,  equant,  sometimes 
fragmentary,  inclusions  few.  Microphenocrysts:  about  35  per  cent,  o.  i  to  0.4  mm.,  anhedral 
equant,  more  or  less  circular  sections,  often  skeleton  forms,  inclusions  few  to  common,  regularly 
arranged. 

Nephelite. — Groundmass:   about  6  per  cent,  anhedral,  interstitial  cement. 

Augite. — Megaphenocrysts:  2  to  o  per  cent,  i  to  3  mm.,  subhedral,  prismatic,  often  frag- 
mentary, greenish-gray.  Microphenocrysts:  about  10  per  cent,  o.i  to  0.3  mm.,  anhedral, 
prismatic,  greenish-gray.  Microgroundmass:  about  20  per  cent,  0.02  to  o.iomm.,  anhedral, 
prismatic  to  equant,  pale  greenish-gray,  arrangement  partly  diverse,  partly  tangential,  around 
the  leucite  microphenocrysts. 

Magnetite. — Groundmass:  about  5  per  cent,  0.02  to  0.05  mm.,  anhedral,  equant. 

Apatite. — Groundmass:   about  i  per  cent,  0.05  to  o.iomm.,  subhedral,  prismatic. 

Glass. — 5  to  o  per  cent,  colorless  to  pale  brown,  usually  absent. 

Chemical  composition  and  norm  as  on  p.  109. 

Type  specimens  from  Crocicchie,  south  of  Lake  Bracciano,  Sabatinian  District. 

II.  7.  2.  2.    Hernical  Braccianose  [Leucitite,  Hernici  Type). 

Megascopic  characters. — Rocks  of  this  type  are  dense,  very  dark  gray  or  almost 
black,  and  either  very  fine-grained  or  quite  aphanitic,  but  without  glassy  luster.  They 
are  typically  aphyric,  phenocrysts  being  either  wholly  wanting,  or  in  some  cases  with 
small  and  rare  ones  of  augite  and  leucite,  which  never  form  more  than  i  or  2  per 
cent  of  the  total  volume.  Vesicular  forms  may  occur,  but  most  of  the  specimens 
are  perfectly  compact.  In  the  field  they  would  be  called  typical  basalts. 

Microscopic  characters. — Under  the  microscope  the  rock  is  seen  to  be  com- 
posed predominantly  of  leucite  and  augite,  with  a  little  feldspar  and  accessory  mag- 
netite and  apatite.  The  texture,  different  from  the  clathrate  one  with  numerous 
leucite  microphenocrysts  of  the  galeral  type,  is  a  xenomorphic  granular  one. 
Neither  the  leucite  nor  the  augite  are  definitely  phenocrystic,  nor  do  either  of  them 
show  much  tendency  to  automorphic  development,  but  of  the  two  the  augite  is  the 
more  automorphic  and  has  a  greater  tendency  to  form  phenocrysts.  Textures 


112  THE  ROMAN  COMAGMATIC  REGION. 

transitional  toward  the  clathrate  one  are  not  uncommon,  so  that  it  is,  at  times,  diffi- 
cult to  say  to  which  type  some  rocks  belong. 

The  leucite,  which  constitutes  about  40  per  cent  of  the  rock,  is  in  subhedral  to 
anhedral  individuals,  sometimes  rounded  and  again  rather  irregular,  interstitial 
between  the  augites.  It  is,  however,  usually  equant,  and  often  contains  small 
inclusions  of  augite,  magnetite,  or  glass,  arranged  zonally.  Skeletal  forms  are  rarely 
seen.  In  size  the  leucites  vary  much,  though  they  are  always  microscopic,  from 
0.05  to  0.30  mm.,  some  of  the  more  irregular  patches  being  larger  than  this  last. 

The  augite,  of  the  usual  pale-gray  color,  is  almost  as  abundant  as  the  leucite 
and  is  phenocrystic  to  some  extent,  in  subhedral  prismoids,  often  "corroded"  and 
with  irregular  outlines,  of  from  0.5  to  i  mm.  in  length.  But  for  the  most  part  it 
forms  subhedral  to  quite  anhedral  individuals,  either  stout  prismoids  or  irregular 
grains  and  interstitial  patches  between  the  leucites.  These  augites  are  of  dimen- 
sions commensurate  with  the  leucites,  from  o.i  to  0.5  mm.  long,  and  they  are  much 
stouter,  larger,  and  more  equantly  developed  than  the  small  augites  of  the  galeral 
type,  in  which  the  augite  form  is  typically  slender  prismatic.  Furthermore,  they 
lack  the  felted  arrangement  and  tangential  position  toward  the  leucites  of  this  last 
type.  With  the  above  minerals  are  small  grains  of  olivine  and  magnetite  and  very 
small  apatite  needles. 

Lying  between  the  larger  leucites  and  augites,  and  inclosing  the  smaller  crystals, 
is  a  colorless  cement  which  the  use  of  crossed  nicols  resolves  into  three  distinct  sub- 
stances. Part  of  it  is  isotropic,  of  a  slightly  lower  refractive  index  than  the  leucite, 
and  may  be  considered  a  glass.  Again,  it  is  feebly  doubly  refracting  without 
twinning  lamellae,  probably  nephelite,  the  presence  of  which  is  indicated  by  the 
calculation  of  the  mode  from  the  norm.  Elsewhere  this  base  consists  of  anhedral 
individuals  of  feldspar,  the  irregular  patches  of  which  extinguish  simultaneously 
over  considerable  areas,  and  which  inclose  poikilitically  the  smaller  leucites  and 
augites.  A  small  proportion  of  this  feldspar  is  orthoclase,  distinguished  by  its 
refractive  index  and  by  the  simple  Carlsbad  twinning;  but  most  of  it  is  a  multiply- 
twinned  labradorite,  of  about  the  composition  A^Anj. 

The  origin  of  these  poikilitic  feldspars  is  somewhat  problematical.  In  most 
of  my  specimens,  especially  those  from  Arcioni  (one  of  which  was  chosen  for  analy- 
sis), I  can  not  but  regard  it  as  primary,  that  is,  one  of  the  last  products  of  crystalliza- 
tion, but  antecedent  to  the  nephelite  and  glass  base.  On  the  other  hand,  Sabatini* 
and  Viola,f  who  have  studied  it,  the  former  in  the  Latian  lavas  and  the  latter  in  those 
of  the  Hernican  District,  regard  it  as  secondary.  This  conclusion  is  based  on  the 
observations  made  by  them  that  many  individuals  with  the  characteristic  forms  and 
inclusions  of  leucite  show,  between  crossed  nicols,  the  birefringence  and  multiple  twin- 
ning of  plagioclase,  which  varies  in  different  cases  from  an  oligoclase  to  anorthite. 
The  twinning  lamellae  pass  out  uninterruptedly  from  the  apparent  leucite  crystal 
into  a  surrounding  colorless  patch  of  feldspar,  of  identical  optical  properties,  extin- 

*  V.  Sabatini,  Boll.  Soc.  Geol.  Ital.,  X  (1896),  p.  70,  also  Mem.  descr.  Carlo  Geol.  Ital.,  X,  1900,  pp.  155,  263,  276. 
t  C.  Viola,  Bott.  Com.  Geol.  Ital.,  1896,  pp.  14,  23;  Proc.  Soc.  Tosc.,  1896,  p.  i;  Neu.  Jahrb.   1899,  I,  pp.  127,  131. 


.PETROGRAPHY. 


guishing  as  a  whole  or  in  separate  twinning  lamellae  simultaneously  over  consider- 
able areas,  which  inclose  poikilitically  small  augites  and  magnetites.  These  obser- 
vations I  have  been  able  to  confirm  in  sections  kindly  shown  me  by  Signor  Sabatini, 
but  have  been  unable  to  discover  any  such  cases  in  the  specimens  collected  by  myself, 
either  from  the  Latian  or  Hernican  districts.  In  all  my  specimens  the  leucite  areas 
are  dark  between  crossed  nicols,  or  show  the  characteristic  twinned  structure,  and 
interrupt  the  continuity  of  the  lamellae  of  the  inclosing  feldspar. 

Chemical  composition. — An  analysis  of  this  type  was  made  on  a  specimen  from 
Gli  Arcioni,  near  Rocca  di  Papa,*  in  the  Latian  District,  and  is  as  yet  unpublished. 
Chemical  Composition  of  Hernical  Braccianose  [Leucitite]. 


I. 

II. 

III. 

SiO3  

47.  20 

0.787 

47.80     o 

708 

47.  6C 

A12O,3    

17.66 

.  173 

17.87 

I7C 

18.1* 

Fe,O*   . 

5.  Cl 

.022 

4.Q3 

031 

2.6? 

FeO  

4-  ">O 

.063 

3.64 

oco 

6.48 

MeO  .  . 

4.20 

.  IOC 

3.68 

002 

4-  10 

CaO  

O.  C2 

.  1  70 

8.70 

ICC 

9  .OI 

Na2O  

2  .  2C 

.036 

2.60 

042 

2.78 

KaO  

7.6? 

.081 

8.2* 

087 

7-47 

HaO+  

O.  72 

o.6c 

o.  n 

H2O-  

O.  C7 

O.  II 

CO2  

none 

none 

none 

TiO2  

I.  10 

.QIC 

o.  77 

•  OIO 

I.  I? 

ZrO2  

0.04 

O.O2 

O.O2 

PjOr.    . 

o.c8 

.004 

o.  36 

.OO^ 

O.  CO 

SO, 

none 

none 

none 

s  

none 

none 

none 

MnO  

n.d. 

n.d. 

n.d. 

BaO  

o.  10 

0.28 

0.24 

99.76 

99.68 

100.47 

Sp.gr. 

2.781 

I.  Hernical    braccianose  [leucitite].     Arcioni,  Rocca  di  Papa,  Latian  District.     Washington, 
analyst. 

II.  Galeral  braccianose  [leucitite].     Near  Crocicchie,   south  of  Lake  Bracciano,   Sabatinian 

District.     Cf.  p.  109. 

III.  Vesbal  braccianose  [leucite-tephrite].     Lava  of  1872,  Mount  Vesuvius.     Cf.  p.  104. 

Ratios  of  I. 
Sal 
Fern 


Norm  of  I. 

Or 17-24?          8 

An I5-57S    3 

Lc 21.80 

Ne 10.22 

Di 22.46 

II 2 . 29 

Mt 5 . 10 

Ol 2.28 

Ap 1.34 


32.02 
24-75 


i-34 


64.83 


33-47 


Class . 


•i-94 


Order . 


Rest. 


Rang 

Subrang. . 

99.82 
*  Cf.  Sabatini,  Mem.  dr.scr.  Carlo  Geol.  Ital.,  X,  1900,  p.  260,  fig.  58. 


98.30 
•    1-52 


F 
L 

K2O'  +  Na2O' 
CaO' 

K2O' 


Na,O' 


=  2.08 


=  2.25 


IH  THE  ROMAN  COMAGMATIC  REGION. 

The  above  analysis  (I)  calls  for  no  comment,  except  to  point  out  its  great  gen- 
eral resemblance  to  those  of  the  other  types  of  braccianose.  It  may  also  be  noted 
that,  while  the  total  amounts  of  the  two  oxides  of  iron  are  about  the  same  in  all  of 
the  types,  their  relative  amounts  are  distinctly  different  in  the  galeral  and  hernical 
from  what  they  are  in  the  types  at  Vesuvius,  where  they  are  very  uniform,  ferrous 
oxide  being  here  largely  in  excess  over  ferric. 

Mode. — As  the  rock  was  relatively  large-grained,  compared  with  the  ground- 
masses  of  the  other  types  of  the  same  subrang,  and  as  the  size  and  equant  shape  of 
the  crystals  made  it  seem  probable  that  the  greater  part  of  them  would  extend  through 
the  section,  an  attempt  at  measuring  the  mode  under  the  microscope  was  made,  with 
the  result  shown  below.  In  this  the  leucite,  feldspars,  and  nephelite  and  glass  were 
necessarily  measured  together,  as  proper  discrimination  between  them  was  difficult. 
The  mode  was  also  calculated  in  the  usual  way  from  the  norm. 


CALCULATED. 


MEASURED. 


Orthoclase,  Or3Ab2 . . 
Labradorite,  AbiAn3. 

Leucite 

Nephelite 

Augite 

Olivine 

Magnetite 

Apatite 


6-5 
IS  o 
32-5 

6.8J 
32.0 

2-3 

2.6 


Vol.  %.     Sp.  gr.  Wt.  %. 

61.0   X   2.6   =  158.0  54.2 

33-9   X  3-3   -  ni-9  38-4 

2-5    X  3.3   =  8.3  2.8 

2.6   X  5.2   =  13.5  4.6 


291.7     100.0 


Evidently  the  influence  of  overlapping  has  been  quite  serious,  being  especially 
marked  relatively  in  the  case  of  the  small  magnetite  grains.  Of  the  two  the 
calculated  mode  is  the  more  authoritative. 

Occurrence. — This  type  is  abundant  in  the  Latian  District,  as  near  Grotta  Fer- 
rata  and  Rocca  di  Papa,  and  in  the  Hernican  District,  two  typical  localities  in  this 
being  the  Torre  di  Pidocchia  near  Frosinone  and  blocks  in  tuff  near  the  Casa  Co- 
lumbella  near  Pofi.  It  was  also  found  in  the  Vulsinian  District,  especially  near 
Montefiascone,  as  at  the  railroad  tunnel  and  at  Madonna  d'Oro.  It  also  probably 
occurs  in  the  Sabatinian  District,  though  my  specimens  of  braccianose  rocks  from 
here  are  to  be  referred  mostly  to  the  galeral  type,  in  which  the  leucite  is  phenocrystic 
rather  than  the  augite. 

Name. — The  type  name  is  derived  from  that  of  the  Hernican  District,  where  it 
seems  to  be  common. 

In  the  prevailing  systems  of  classification  these  rocks  would  be  called  leucitites, 
like  the  preceding  type,  as  the  amount  of  feldspar  is  very  small,  though  in  some  cases 
the  name  of  leucite-tephrite  might  be  applicable. 


PETROGRAPHY.  115 

MERNICAL  BRACCIANOSE.    II.  7.  2.  2. 

Megascopic  characters. — Very  dark  gray  or  black,  compact,  aphyric,  or  only  slightly  por- 
phyritic.  Augite  phenocrysts  rare,  i  to  3  mm.,  prismatic,  black.  Leucite  phenocrysts  very 
rare,  i  to  4  mm.,  equant,  white  or  pale  gray,  inconspicuous.  Groundmass  very  fine-grained  or 
aphanitic. 

Microscopic  characters. — Holocrystalline,  aphyric,  few  or  no  distinct  phenocrysts,  xeno- 
morphic  granular  fabric,  leucite,  augite,  labradorite,  orthoclase,  nephelite,  olivine,  magnetite, 
apatite. 

Soda-orthoclase,  Or3Ab2. — About  5  per  cent,  anhedral,  as  formless  interstitial  patches. 

Labradorite,  AbrAn3. — About  15  per  cent,  anhedral,  formless  interstitial  patches,  multiply 
twinned. 

Leucite. — 30  to  35  per  cent,  0.05  to  0.30  mm.,  anhedral  equant,  less  often  irregular,  inter- 
stitial between  the  augites,  inclusions  few,  skeleton  forms  very  rare. 

Nephelite. — Groundmass:   about  5  per  cent,  anhedral,  formless,  interstitial  patches. 

Augite. — 30  to  35  per  cent,  o.  10  to  0.50  mm.,  subhedral  to  anhedral,  prismatic  to  equant, 
and  irregular,  greenish-gray,  inclusions  few. 

Olivine. — About  2  per  cent,  0.05  to  o.  10  mm.,  anhedral,  equant. 

Magnetite. — About  3  per  cent,  0.02    to  0.05  mm.,  anhedral,  equant. 

Apatite. — About  i  per  cent,  0.05  to  o.iomm.,  subhedral,  prismatic. 

Chemical  composition  and  norm  as  on  p.   113. 

Type  specimen  from  Arcioni,  Rocca  di  Papa,  Latian  District. 

II.  7.  2.  2.    Atrial  Braccianose  [Leucite-Tephrite,  Atrio  Type]. 

Megascopic  characters. — This  type,  which  must  be  regarded  as  much  less  pre- 
cisely characterized  than  the  preceding  ones,  occurs  only  as  very  small  flows  and  as 
the  borders  and  surfaces  of  larger  ones,  salbands  of  dikes,  and  crusts  of  bombs.  The 
rocks  are  black  or  a  very  deep  brown,  evidently  highly  vitreous,  and  sprinkled  with 
numerous  small  round  spots  of  leucite,  with  only  rare  and  small  phenocrysts  of  feld- 
spar, augite,  and  occasionally  olivine,  the  last  only  as  an  accessory.  Vesicular  and 
scoriaceous  forms  are  common. 

Microscopic  characters. — In  thin  section  these  rocks  show  many  small  (up  to  i  or 
2  mm.),  highly  euhedral  and  trapezohedral  sections  of  leucite,  sometimes  with  and 
sometimes  without  inclusions  of  glass,  etc.  Leucite  skeleton  forms  are  very  common. 
Leucite  makes  up  about  one-quarter  of  the  rock  volume,  but  the  relative  amount  may 
vary  rather  widely.  Tables  and  prisms  of  labradorite  (from  Ab2An3  to  AbtAn3)  are 
present  in  less  amount,  not  over  10  per  cent,  and  also  more  or  less  euhedral  pheno- 
crysts of  greenish-gray  augite,  which  are  apt  to  be  equant  rather  than  prismoidal. 
The  amount  of  these  is  small,  about  that  of  the  feldspars.  There  may  be  also  a  few 
olivines  and  magnetite  grains,  but  always  less  than  the  two  immediately  preceding 
minerals.  All  these  lie  in  a  glass  base,  which  is  dark  brown  in  color,  either  clear  or 
dusty  with  indeterminable  microlites,  and  which  frequently  shows  flow  textures. 

Chemical  composition. — An  analysis  was  made  of  a  specimen  of  this  type,  the 
material  having  been  kindly  furnished  by  Professor  G.  Mercalli,  of  Naples,  who  has 
published  it  elsewhere. 


n6 


THE  ROMAN  COMAGMATIC  REGION. 

Chemical  Composition  of  A  trial  Braccianose  [Leucite-tephrite]. 


I. 

I 

SiO2  

48.  10     o 

802 

CO2  

none 

A12O3  

17.  ^6 

1  72 

TiO2  

o  018 

Fe2O3  

2.48 

016 

ZrO2  

trace 

FeO  

6.  10 

08  c 

P2O<  .  . 

OO7 

MeO... 

4.  27 

IO7 

SO3  

trace 

CaO     

8.16 

i<±6 

s  

Na2O     

2.65 

OJ.J. 

CuO  

K2O      

7.Q3 

084 

MnO  

n  d 

H2O+         

0.  12 

BaO     . 

o  08 

HaO 

99.91 

I.  Atrial  braccianose  [leucite-tephrite  obsidian].  Lava  of  August  27,  1903,  Valle  dell'  Inferno, 
Mount  Vesuvius.  Washington,  analyst.  Cf.  G.  Mercalll,  Boll.  Soc.  Sism.  Ital., 
X,  1904,  p.  21. 


66.94 


32.91 


Or    

Norm. 

.   2S.  86 

0               \ 

An   

.     12.  21* 

38-09) 

Lc        .    .  .  . 

.   i6.3<  ' 

f 

Ne        .      . 

.     12.  ^O 

28.85  ) 

Di  

.  17.80 

01  

6.26 

24-IS 

Mt    

3.  71 

11        .... 

2.  74 

6-45 

AD 

2.  31 

2.  31 

99-85 

Rest 0.24 


Ratios. 

Class  

Sal 

OJ 

Order  

Fern 
F 

72 

"L 
KO'+Na2O' 

Subrane.  . 

CaO' 
K2O' 

.91 

ni 

100. og 

Of  this  analysis  it  need  only  be  remarked  that  it  resembles  very  closely  those  of 
the  two  preceding  types. 

Mode. — As  the  type  is  very  highly  vitreous,  and  therefore  indeterminate,  any 
exact  statement  of  the  mode  is  uncalled  for.  It  will  be  of  interest  to  state  the  rela- 
tive amounts  of  the  various  minerals  and  glass,  which  are  so  easily  estimated,  the 
statement  being  based,  of  course,  on  the  specimen  analyzed.  These  figures  may 
vary  widely  in  other  specimens. 


Vol.  %.      Sp.  gr. 

Leucite 26. 4  X  2.5 

Labradorite 5.9    X   2.6 

Augite 4-5   X  3-3 

Magnetite 0.6   X  5.2 

Glass 62.6   X  2.7 


66.0 

15-3 

14-9 

3-i 

169.0 


Wt.%. 
24.6 

5-7 

5-6 

i.i 

63.0 


268.3     100.0 


Occurrence.  —  As  already  stated,  this  type  occurs  only  as  flows  of  very  small 
volume,  as  well  as  borders  of  larger  flows  and  crusts  of  bombs  at  Mount  Vesuvius, 
and  also  as  narrow  dikes  and  salbands  of  thick  ones  at  Monte  Somma.  It  has  not 
been  observed  elsewhere  in  the  Roman  Region. 


PETROGRAPHY.  117 

Name. — The  name  of  the  type  is  derived  from  the  Atrio  del  Cavallo,  the  semi- 
circular valley  between  Vesuvius  and  Somma,  into  which  so  many  of  the  small  flows 
of  the  present  cone  run,  and  from  the  southern  part  of  which  the  analyzed  specimen 
was  obtained. 

In  the  prevailing  systems  the  name  of  leucite-tephrite-obsidian  is  undoubtedly 
the  correct  appellation. 

ATRIAL  BRACCIANO5B.    il.  7.  2.  2. 

Megascopic  characters. — Very  dark  gray  or  black,  sprinkled  with  small  white  spots,  compact 
to  vesicular,  highly  porphyritic.  Leucite  phenocrysts  abundant,  i  to  3  mm.,  round,  white. 
Augite  phenocrysts,  very  few,  i  to  2  mm.,  prismatic,  black,  inconspicuous.  Feldspar  and  olivine 
phenocrysts  very  rare  or  none,  small.  Groundmass:  black,  vitreous. 

Microscopic  characters. — Hyalocrystalline  to  dohyaline,  porphyritic,  dopatic  to  sempatic. 
Phenocrysts  20  to  40  per  cent,  leucite,  augite,  labradorite,  magnetite,  rarely  olivine.  Ground- 
mass  :  80  to  60  per  cent,  wholly  glass,  sometimes  dusty  with  microlites. 

Labradorite,  Ab2An3  to  AbtAn3. — Phenocrysts:  5  to  10  per  cent,  o.  i  to  0.5  mm.,  euhedral, 
thin  prismatic  or  tabular,  much  twinned,  few  inclusions. 

Leucite. — Phenocrysts:  20  to  30  per  cent,  0.2  to  2.0  mm.,  euhedral  to  subhedral,  often 
clustered,  sometimes  skeleton  forms,  inclusions  few. 

Augite. — Phenocrysts:  5  to  10  per  cent,  0.2  to  i.omm.,  euhedral  to  subhedral,  equant 
to  prismatic,  greenish-gray,  few  inclusions. 

Magnetite. — Phenocrysts:   about  i  per  cent,  0.02  to  0.05  mm.,  anhedral  or  skeletal. 

Glass. — Yellowish-brown,  often  clear,  sometimes  mottled,  sometimes  dusty  with  indeter- 
minable microlites. 

Chemical  composition  and  norm  as  on  p.  116. 

Type  specimen  from  flow  of  1903,  Valle  del  Inferno,  Mount  Vesuvius,  Campanian  District. 

II.  8-7.  2.  2.    Scalal  Vesuvose-Braccianose  [Leucite-Tephrite,  Scala  Type]. 

Megascopic  characters. — This  type  is  porphyritic,  but  not  as  much  so  as  in  those 
just  described.  The  phenocrysts  are  mostly  of  augite,  with  rare  ones  of  olivine,  but 
few  or  none  of  leucite,  and  do  not  make  up  more  than  about  10  per  cent  of  the  rock 
volume.  The  augites  are  from  2  to  5  mm.  long,  and  prismatic  rather  than  equant, 
the  color  a  dark  green.  The  rare  olivines  are  about  the  same  size,  equant,  and  pale 
greenish-yellow.  The  groundmass  in  this  type  is  a  rather  light  gray,  not  nearly  as 
dark  as  the  preceding  rocks,  and  is  phanerocrystalline  and  fine-grained,  the  dark 
and  light  mineral  particles  being  easily  distinguishable  with  a  lens.  In  the  field 
rocks  of  this  type  would  be  called  augite-leucophyres. 

Microscopic  characters. — The  augite  phenocrysts  show  subhedral  and  often 
fragmentary  forms,  and  are  of  the  usual  very  pale-gray  color  and  quite  free  from 
inclusions.  The  very  rare  olivines  present  no  features  of  special  interest,  but  are 
not  well  developed  crystallographically.  An  occasional  leucite  phenocryst  is  seen 
here  and  there,  but  these  are  as  rare  as  are  phenocrysts  of  augite  in  the  vesbal  type. 

The  groundmass  is  holocrystalline  typically,  though  a  trifling  amount  of  glass 
may  rarely  be  present.  It  is  microporphyritic,  small  phenocrysts  of  leucite  and 
augite  lying  in  a  microgroundmass  which  resembles  the  preceding  in  its  intersertal 
fabric.  The  leucite  microphenocrysts  are  very  abundant,  as  is  expected,  since  it 
does  not  occur  as  megaphenocrysts.  They  are  anhedral  and  mostly  spheroidal, 


n8 


THE  ROMAN  COMAGMATIC  REGION. 


though  some  irregular  forms  are  noticed.  They  do  not  show  very  marked  birefrin- 
gence, and  inclusions  are  few  and  irregularly  arranged.  The  microphenocrysts  of 
augite  are  anhedral  to  subhedral,  generally  prismatic,  and  of  the  same  color  as  the 
larger  ones. 

The  microgroundmass  between  these  is  composed  of  small  tables  of  labradorite, 
with  twinning  and  extinction  angles  which  correspond  to  the  composition  Ab2An3, 
small  irregular  grains  of  augite  and  fewer  of  olivine,  and  the  usual  magnetite  grains 
and  apatite  needles,  neither  of  which  are  numerous.  The  arrangement  of  the  feld- 
spar tables  is  diverse,  inclosing  the  other  constituents.  Between  these  well-defined 
minerals  is  a  colorless  residual  cement,  which  in  most  specimens  is  somewhat  bire- 
fringent  and  to  be  regarded  as  nephelite,  which  treatment  of  the  rock  powder  with 
weak  acid  proves  to  be  present,  and  there  is  occasionally  an  isotropic  glass  of  similar 
composition. 

Chemical  composition. — An  analysis  of  this  type  was  made  from  material  ob- 
tained at  the  quarries  in  the  great  flow  of  1631,  at  La  Scala,  near  Torre  del  Greco, 
Mount  Vesuvius.  It  has  been  already  published,  but  in  incomplete  form  and  with- 
out any  petrographic  details.  For  comparison  are  also  given  two  of  the  older  analy- 
ses of  the  same  rock. 

Chemical  Composition  of  Scalal  Vesuvose-braccianose  [Leucite-lephrite]. 


I. 

II. 

III. 

SiO2  

4.7.71      O 

70  c 

4.7.4.7     o 

701 

46  .  4.  1       O 

A12O3  

17.61 

173 

16.69 

164. 

IQ.67 

103 

Fe2O3   

2.46 

ore 

4..  2O 

026 

6.88 

O4.3 

FeO  

<;.68 

070 

"?  -QO 

082 

4.  17 

o<;8 

MgO  

4.80 

1  20 

4..  34. 

109 

?.  23 

131 

CaO  

0  .42 

1  68 

O.o8 

I7O 

10.53 

1  88 

Na2O  

2  .  7? 

O4.C 

2.28 

O37 

2.  O2 

032 

K2O  

7.64, 

08  1 

7   4.6 

080 

4OO 

(X3 

HaO  +  

f  •«*» 

trace    / 

H2O  -  

n.  d.*  \ 

0.08 

O.  II 

coa  

none 

n.  d. 

n.  d. 

TiO2  

O.  37 

ooc 

O  .  23 

OO3 

n  d. 

ZrO2  

0.06 

n.  d. 

n.  d. 

P2Oc  

O.  77 

ooc 

O.  IQ 

OOI 

n.  d. 

SO,  

none 

n.  d. 

n.  d. 

Cl  

O.  4.O 

OI  I 

O.  4.1 

Oil 

s  

none 

n  d 

n  d 

MnO  

n.  d. 

I     1C       O 

16 

n  d 

BaO  

o.  26 

n  d 

n  d 

99-53 

100.69 

100.42 

So.  er.  . 

*  Material  dried  at  110°. 

I.  Scalal  vesuvose-braccianose  [leucite-tephrite].     La  Scala,  Torre  del  Greco,  Mount  Vesu- 
vius.    Washington,  analyst.     Cf.  Washington,  Prof.  Paper  U.  S.  G.  S.  No.  14, 

^oa.  P-  3°7- 
II.  Scalal  vesuvose-braccianose  [leucite-tephrite].     Same  locality.     Haughton,  analyst.    Trans. 

R.  Irish  Acad.,  XXVI,  1876,  p.  77. 
III.  Scalal  braccianose  [leucite-tephrite].     Same  locality.     Fuchs,  analyst.  Neu.  Jahrb.,  1866, 
p.  678. 


j,  0? 

Lc 

26  60 

Ne    

12  78 

Di  

2,    i- 

01  

6  o? 

Mt      

.    -1.48 

Jl        

.   o.  76 

Ap      

.    1  .  77 

Rest 

99.26 

.     O    72 

PETROGRAPHY.  1 19 

Norm  of  I.  Ratios  of  I. 

1 . 65  Fern 

29.60  Order'"  -T  =I-62 

«•*    3S'6'         Rang S 

i-77 


K,O' 
Subrang  ...............  -'  =I-8° 


99-58 


Comparison  of  this  analysis  with  that  of  the  vesbal  type  shows  that  the  two  are 
remarkably  similar,  the  figures  for  silica  and  the  alkalis  especially  being  nearly 
duplicates.  The  ratios,  however,  show  that  this  type  is  closer  to  the  border  of  order 
8  than  the  other,  so  close  that  it  should  be  regarded  as  transitional  between  brac- 
cianose  where  it  actually  falls,  and  vesuvose,  the  homologous  subrang  in  order  8  of 
dosalane.  In  fact  this  type  was  formerly  assigned  to  the  subrang  of  vesuvose*  on 
the  basis  of  the  incomplete  analysis,  and  the  change  in  position  is  due  to  the  deter- 
mination of  P2OS,  which,  by  combining  with  CaO  to  form  apatite,  sets  free  sufficient 
SiO2  to  change  the  relative  amounts  of  orthoclase  and  leucite  so  that  the  ratio  of 
feldspar  to  lenads  is  lenfelic  rather  than  feldolenic. 

Of  the  two  older  analyses,  that  of  Haughton  (II)  most  closely  resembles  the 
latest  one,  and  must  be  regarded  as  the  more  correct.  The  only  differences  of  note 
are  the  lower  alumina  and  the  very  high  figure  for  MnO.  This  last  is  undoubtedly 
erroneous,  as  the  amount  of  this  constituent  should  not  exceed  0.20  per  cent,  and  it 
most  probably  arises  from  the  common  analytical  mishap  of  incomplete  precipitation 
of  alumina,  the  unprecipitated  portion  of  which  is  subsequently  thrown  down  with 
the  manganese  and  weighed  as  such.  If  this  is  assumed  to  be  true  and  the  apparent 
amount  of  MnO  is  added  to  that  of  A12O3,  the  figure  for  the  latter  becomes  17.82, 
in  close  agreement  with  I. 

Of  analysis  III  little  need  be  said,  except  that  it  is  undoubtedly  incorrect  in  the 
figures  for  alkalis,  especially  potash.  The  amount  shown  allows  of  the  formation 
of  only  23.1  per  cent  of  leucite,  which  is  much  below  the  proportion  which  the  micro- 
scope shows  is  present,  and  would  lead  to  the  presence  of  about  40  per  cent  of  labra- 
dorite,  AbjArij,  a  figure,  on  the  other  hand,  much  in  excess  of  the  actual  amount. 

Mode.  —  The  estimation  of  the  mode  of  this  type  is  strictly  analogous  to  that  of 
the  vesbal.  While  the  augite  megaphenocrysts  could  be  estimated  readily,  and  the 
microphenocrysts  of  leucite  fairly  well,  the  other  constituents  did  not  lend  them- 
selves to  this  method  of  procedure.  On  the  other  hand,  the  calculation  of  the  mode 
from  the  norm  presents  no  difficulty,  and  it  is  noteworthy  that  here,  as  in  the  vesbal 
type,  the  silica  left  available  for  soda  after  formation  of  augite,  olivine,  leucite,  and 

*  See  reference  under  the  analysis. 


120  THE  ROMAN  COMAGMATIC  REGION. 

anorthite  is  just  sufficient  to  yield  the  right  amount  of  albite  for  the  observed  com- 
position of  the  labradorite,  using  the  regular  equations. 

Labradorite,  Ab2An3 13.8 

Leucite 35-6 

Nephelite 9.9 

Augite 31.8 

Olivine 6.1 

Ores i .  o 

Apatite 1.8 

100.0 

This  result  is  very  similar  to  the  mode  of  the  vesbal  type,  and  is  in  harmonj 
with  what  we  have  noted  of  the  rock  characters.  The  amount  of  labradorite  is  lower 
and  that  of  leucite  higher  in  accord  with  its  transitional  magmatic  position,  and  the 
slightly  greater  quantities  of  augite  and  olivine  and  less  of  ores  agree  with  the  micro- 
scopic observations.  The  presence  of  considerable  nephelite  was  confirmed  by 
treatment  with  very  dilute  acid,  as  in  the  previous  case. 

Occurrence. — Rocks  of  this  type  are  only  met  with  in  the  Campanian  District, 
especially  at  Mount  Vesuvius.  Here  they  are  most  typically  represented  by  the 
great  flows  of  1631,  which  reached  the  sea  at  various  points  between  Portici  anc 
Torre  Annunziata.  These  are  quarried  extensively  for  use  in  Naples,  especially  at 
Granatello  near  Portici  and  La  Scala  near  Torre  del  Greco.  Specimens  from  the 
following  flows,  which  have  been  examined  by  me,  also  belong  to  this  type:  1551, 
1555,  1806,  1810,  1822,  1855.  The  descriptions  of  Fuchs  and  others  do  not  allo\ 
of  discrimination  between  this  type  and  another  with  more  abundant  and  larger 
augite  megaphenocrysts. 

Name. — The  type  derives  its  name  from  the  locality,  La  Scala,  of  the  quarry 
whence  the  material  analyzed  was  derived.  In  prevailing  classifications  it  would  be 
known  as  a  leucite-tephrite,  as  in  the  preceding  types. 

SCALAL  VESUVOSE-BRACCIANOSE.    II.  8-7.  2.  2. 

Megascopic  characters. — Light  gray,  compact,  somewhat  porphyritic.  Augite  pheno- 
crysts  not  abundant,  2  to  5  mm.  long,  prismatic,  dark-green  or  black.  Leucite  phenocrysts  very 
rare  or  none,  i  to  3  mm.,  round,  pale-gray  or  white.  Olivine  phenocrysts  very  few  or  none< 
0.5  to  2  mm.,  equant,  pale  greenish-yellow.  Groundmass,  light  gray,  very  fine-grained,  phanero- 
crystalline. 

Microscopic  characters. — Holocrystalline,  mediophyric,  perpatic.  Phenocrysts:  about  10 
per  cent,  augite,  sometimes  also  leucite  and  olivine.  Groundmass,  about  90  per  cent,  micro- 
porphyritic.  Microphenocrysts :  about  50  per  cent,  leucite,  augite.  Microgroundmass :  about  40 
per  cent,  labradorite,  augite,  nephelite,  magnetite,  olivine,  apatite. 

Labradorite,  Ab2An3. — Groundmass:  about  14  per  cent,  0.05  to  0.20  mm.,  subhedral, 
thin  tabular  or  prismatic,  twinned,  arrangement  diverse. 

Leucite. — Phenocrysts:  none  to  2  per  cent  0.5  to  2.0  mm.,  anhedral,  equant  to  irregular. 
Microphenocrysts:  about  35  percent,  o.  1 100.3  mm.,  anhedral,  equant  to  irregular,  inclusions  few. 

Nephelite. — Groundmass:    about  5  to  10  per  cent,  anhedral,  as  interstitial  cement. 

Augite. — Megaphenocrysts:  about  10  per  cent,  2  to  5  mm.,  subhedral,  prismatic,  some- 
times fragmentary,  greenish-gray,  few  inclusions.  Microphenocrysts:  about  10  per  cent,  o.  i  to 
0.4 mm.,  subhedral  to  anhedral,  prismatic  to  equant,  pale  greenish-gray.  Microgroundmass: 
about  10  per  cent,  0.02  to  o.iomm.,  anhedral,  equant. 


PETROGRAPHY.  121 

Olivine. — Phenocrysts:  none  to  2  percent,  0.5  to  2.0  mm.,  subhedral  to  anhedral,  equant 
to  fusiform,  colorless.  Groundmass:  about  4  per  cent,  o.  2  to  0.02  mm.,  anhedral,  equant. 

Magnetite. — Groundmass:  about  2  per  cent,  0.02  to  0.05  mm.,  anhedral,  equant. 

Apatite. — Groundmass:  about  2  per  cent,  0.05  to  i.omm.,  subhedral,  prismatic. 

Chemical  composition  and  norm  as  on  p.  ng. 

Type  specimen  from  flow  of  1631,  La  Scala,  near  Torre  del  Greco,  Mount  Vesuvius, 
Campanian  District. 

II.  8.  2.  2.     Romal  Vesuvose  [Leucitite,  Rome  Type]. 

This  type  is  as  yet  unestablished  by  an  analysis,  so  that  a  brief  description  of 
it  must  suffice.  It  is  based  on  several  specimens  which,  on  careful  comparison  with 
those  of  similar  types  of  undoubted  braccianose  and  albanose  rocks,  were  judged 
to  be  intermediate  between  them,  differing  from  the  former  in  the  total  absence  of 
feldspar  between  the  leucite  and  small  augite  crystals,  nephelite  or  glass  in  small 
amount  replacing  it,  and  from  the  latter  by  the  considerably  greater  amount  of 
leucite  as  compared  with  augite,  indicating  a  position  in  dosalane  rather  than  in 
salfemane.  These  measurements  could  not  be  carried  out  very  exactly,  for  reasons 
given  above,  but  the  differences  are  clearly  seen  on  comparison  of  the  sections.  It 
may  also  be  added  that  the  occurrence  of  such  a  type  is  to  be  expected  in  the  region. 

Megascopic  characters. — The  rocks  are  very  dark  gray  or  almost  black,  dense 
and  compact,  showing  very  few  phenocrysts  of  leucite  and  augite.  The  groundmass 
is  aphanitic,  or  extremely  fine-grained. 

Microscopic  characters. — These  correspond  in  every  way  with  those  described 
later  under  romal  albanose,  except  that  leucite  is  slightly  more  abundant.  The 
fabric  is  in  most  cases  a  clathrate  one,  composed  of  numerous  small,  round  anhedra 
of  leucite,  with  some  pale  greenish-gray  augite  microphenocrysts,  and  an  interstitial 
microgroundmass  of  small  felted  augite  prisms,  often  tangential  toward  the  leucites, 
with  many  minute  magnetite  grains.  No  feldspar  is  present,  but  between  the  augite 
prismoids  is  a  colorless  cement  which  is  either  nephelite  or  glass.  Olivine  does  not 
often  occur  in  this  type,  but  a  minimal  amount  may  be  present  in  some  cases. 

Chemical  composition  and  mode. — As  no  chemical  analysis  of  this  type  was  avail- 
able, the  attempt  was  made  to  calculate  the  chemical  composition  from  the  measured 
mode.  For  this  a  specimen  was  chosen  from  a  quarry  opposite  the  railroad  station 
of  Oriolo,  below  Monte  Raschio,  in  the  Sabatinian  District,  as  the  amount  of  leu- 
cite seemed  to  be  decidedly  greater  than  in  similar  albanose  rocks,  and  the  grain  was 
sufficiently  large  to  allow  fairly  accurate  measurements  and  with  probably  not  very 
serious  error  from  overlapping  of  the  dark  minerals.  The  results  of  this  are  seen 
below,  in  which  the  "augite"  includes  the  small  amount  of  nephelite  base,  the 
exact  quantity  of  which  is  impossible  to  measure. 

Units  Vol.  %.      Sp.gr.  Wt.  %. 

Leucite 1,041  =  51.0  X  2.5  =  127.5  43-8 

Augite 932  =  45.7   X  3.2*  =  146.2  50.2 

Magnetite 68  =     3.3X5.2  =  17.2  6.0 

2,O4l          IOO.O  290.9       IOO.O 

*3-3  corrected  for  12  per  cent  of  nephelite. 


122  THE  ROMAN  COMAGMATIC  REGION. 

The  rough  figures  given  above  must,  however,  be  corrected  for  the  small  amount 
of  nephelite,  and  some  correction  must  also  be  made  for  the  overlap,  which  we  have 
seen  to  be  a  common  source  of  error  in  all  these  rocks.  The  amount  of  nephelite 
was  assumed  to  be  6  per  cent,  which  is  probably  slightly  below  the  true  figure,  if  we 
can  judge  by  the  analogy  of  the  calculated  modes  of  galeral  and  hernical  braccianose. 
To  estimate  the  amount  of  error  due  to  overlap,  the  calculated  and  measured  modes 
of  hernical  braccianose  (p.  114)  were  taken  as  the  basis,  as  the  two  are  similar  in 
grain.  On  the  assumption  that  the  errors  here  are  about  the  same,  the  observed 
amount  of  leucite  was  increased  by  one-tenth,  that  of  magnetite  reduced  to  one-half, 
and  the  balance  referred  to  augite,  after  subtracting  the  6  per  cent  of  nephelite  from 
this.  The  corrected  mode  would  then  be: 

Leucite  ..........................................  48.  2 

Nephelite  ........................................  6.0 

Augite  ...........................................  42  .  8 

Magnetite  ........................................  3.0 

100.0 

To  calculate  the  chemical  composition  from  this  the  leucite  was  assumed  to 
have  the  composition  of  that  of  Mount  Vesuvius  as  analyzed  by  Steiger,*  the 
nephelite  that  of  Vesuvius  as  analyzed  by  Rauff,f  and  the  augite  that  of  Ticchiena 
as  analyzed  by  myself.^  The  magnetite  was  assumed  to  be  free  from  titanium. 
Using  these  data,  the  chemical  composition  of  the  rock  and  the  norm  are  calculated 
to  be: 

Chemical  Composition.  Norm. 

SiO2  .......................  48.82     0.814         Or  ..................   7.23 


A1203  ......................  16.20  .159  An  ..................      .6 

Fe,03  ......................   4-89  -031  Lc  ..................  40-55,0  « 

FeO  ........................   3.32  .046  Ne  ..................   8.8o)49'35 

MgO  .......................   5.01  .125  Di  ..................  27.00)    , 

CaO  ........................   8.65  .155  Wo  ..................   o.93r7>93L7  ,0 

Na20  .......................    1.89  .031  Mt  ..................   7.19       Q   .J37'4 

K2O  .......................   9.99  .106  II  ...................   2.  28  \    9'47; 

TiOa  .......................   1.24  .015 


This  analysis  resembles  very  closely  those  of  the  braccianose  types  and,  as 
will  be  seen,  those  of  albanose  rocks,  the  chief  difference  being  the  higher  potash. 
Examination  of  the  norm  shows  that  while  it  is  well  within  the  limits  of  order  8,  rang 
2,  and  sub  rang  2,  it  is  almost  exactly  on  the  line  between  dosalane  (in  which  it 
falls)  and  salfemane,  and  is  consequently  an  albanose-vesuvose. 

Occurrence.  —  Rocks  which  may  be  provisionally  referred  to  this  type,  though 
texturally  some  of  them  have  a  fabric  which  is  saccal  rather  than  romal,  are  found 
in  the  Vulsinian  District  at  the  Sassi  Lanciati  and  in  the  Fossa  Melona  on  the  east 
shore  of  Lake  Bolsena,  in  the  Sabatinian  at  Oriolo  and  on  the  north  side  of  Lagosello, 
and  in  the  Auruncan  below  Preta. 

*  Bull.  U.  S.  Geol.  Surv.  No.  220,  1903,  p.  30. 

t  Zeits,  Krysl.,  II,  1878,  p.  454;   Dana,  Mineralogy,  No.  6,  p.  425. 

*  Cf.  p.  134. 


PETROGRAPHY. 


123 


Name. — The  subrang  name  is  derived  from  that  of  Mount  Vesuvius,  on  the 
basis  of  the  position  formerly*  assigned  the  types  of  vesbal  and  scalal  braccianose. 
But,  as  we  have  seen  above,f  these  rocks  do  not  properly  fall  in  vesuvose  (II.  8. 
2.  2),  but  in  braccianose  (II.  7.  2.  2).  The  error  is  an  unfortunate  one  and  the 
subrang  II.  7.  2.  2  could  appropriately  be  called  vesuvose,  as  this  subrang  is  very 
abundant  and  characteristic  at  Vesuvius.  But  on  the  grounds  of  priority  and 
previous  publication,  and  on  account  of  the  liability  to  misunderstanding  if  a 
change  is  made,  it  is  deemed  best  to  preserve  the  names  with  their  original  signifi- 
cations as  to  rang  and  subrang.  The  type  name  will  be  discussed  later, 

In  the  prevailing  systems  of  classification  this  type  would  be  included  among 
the  leucitites,  consisting,  as  the  rocks  do  almost  exclusively,  of  leucite  and  augite. 
III.  8-7.  2.  2.  Galeral  Albanose-Jugose  [  Leucitite,  Galera  Type]. 

Megascopic  characters. — In  the  hand  specimen  this  type  can  not  be  distin- 
guished from  galeral  braccianose,  vesuvose,  or  albanose.  The  rock  is  very  dark 
gray  or  black,  with  rare  phenocrysts  of  leucite  or  augite  in  the  aphanitic  or  very  fine- 
grained groundmass,  and  is  typically  a  basalt,  or  a  melaphyre  if  the  few  phenocrysts 
are  considered  to  be  of  importance.  Vesicular  forms  seem  to  be  uncommon,  and 
all  my  specimens  are  very  compact. 

Microscopic  characters. — In  thin  section  likewise  it  is  a  matter  of  great  difficulty, 
if  not  an  impossibility  in  some  cases,  to  discriminate  between  this  type  and  those 
mentioned  above.  But  in  general  the  greater  amount  of  augite  serves  to  distinguish 
them  from  the  braccianose  and  the  presence  of  small  amounts  of  feldspar  from  the 
albanose. 

In  the  type  specimens  the  leucite  phenocrysts  are  rarely  seen  in  the  section  and 
call  for  no  description.  Those  of  augite  are  more  numerous,  from  0.5  to  i.o  mm. 
long,  stoutly  prismatic  and  subhedral,  and  of  the  usual  very  pale-gray  color,  almost 
wholly  without  inclusions.  The  microgroundmass  shows  a  clathrate  fabric,  with 
abundant  small  round  leucite  anhedra,  weakly  birefringent,  between  which  lie 
many  small  prismoids  and  grains  of  common  augite.  In  the  type  specimen  the 
tangential  arrangement  of  the  prisms  is  not  so  well  marked  as  in  the  galeral  brac- 
cianose from  Crocicchie,  and  the  small  augites  are  rather  more  often  in  the  form  of 
equant  grains,  showing  a  transition  toward  the  hernical  type,  but  the  characteristic 
round  leucites  indicate  that  the  fabric  may  still  be  called  clathrate.  There  are 
many  small  and  irregular  grains  of  magnetite  and  some  olivine  grains  and  apatite 
needles.  Between  these  small  crystals  is  a  colorless  base,  in  part  isotropic,  but  the 
greater  part  of  this  is  doubly  refracting,  with  an  index  of  refraction  and  birefringence 
decidedly  greater  than  leucite,  and  apparently  greater  also  than  nephelite.  Though 
this  shows  no  twinning  lamellae  it  may  be  regarded  as  feldspar,  mostly  plagioclase, 
which  the  calculated  norm  shows  to  be  present,  and  as  treatment  of  the  rock  powder 
with  dilute  acid  furnished  only  very  small  amounts  of  gelatinous  silica.  In  some  of 

*  Washington,  Prof.  Paper  U.  S.  Geol.  Surv.  No.  14,  1903,  p.  307. 
t  Cf.  pp.  104  and  119. 


124 


THE  ROMAN  COMAGMATIC  REGION. 


the  sections  there  is  a  very  small  amount  of  yellowish  melilite  in  interstitial  patches, 
marking  a  transition  toward  the  boval  type  to  be  described  later,  but  this  mineral 
here  is  quite  negligible.  Glass  does  not  seem  to  be  present,  at  least  to  a  notable 
extent. 

Chemical  composition. — An  analysis  of  this  type  was  made  from  the  quarry  at 
Monte  Jugo,  the  specimen  being  selected  from  this  small  flow  so  as  to  furnish  a  basis 
of  comparison  with  Ricciardi's  analysis,  without  chance  of  great  differences  in  the 
results  due  to  variations  in  the  mass  of  rock.  For  this  purpose  his  analysis  of  the 
same  rock  is  also  given  in  II. 

Chemical  Composition  of  Galeral  Albanose-jugose  [Leucitite]. 


I. 

II. 

I. 

II. 

SiO3  

A.1  .  7O     O.  7OO 

A.8.  7O     O.  8oC 

TiOj  .... 

A13O3  

14..  70        .Id? 

ic.o7       .148 

ZrO2  

O.O4 

Fe2O3  

3.  IO        .OIQ 

I  .  ?  3         .  OOO 

P»O<  .  . 

O./lC      O.OO2 

O.47     O.OO7 

FeO  

c.o8      .071 

9.18      .128 

SO,  .  . 

none 

MeO    . 

6.  77       .  160 

7.48      .187 

s  

none 

CaO  

11.61      .207 

I3.Q?         .240 

(Ce,Di),O,. 

o.o< 

NaaO  

1  .  4.0        .  024. 

O.O4.        .OI? 

MnO  

n.d. 

O.  2O 

K2O  

6.O7        .  O72 

I  .  73        .Ol8 

BaO  

O.  I? 

H2O+  

O.  77  1 

SrO  

0.04 

Hf) 

n     ~Q    f 

1.78 

coa  

none 

100.35 

100.72 

I.  Galeral  albanose-jugose  [leucitite].     Monte  Jugo,  south  of  Montefiascone,  Vulsinian  District. 

Washington,  analyst. 
II.  Same  rock.     Same  locality.  Ricciardi,  analyst.  Cf.  Klein,  Neu.  Jahrb.,  B.  B.  VI,  1889,  p.  20. 


=  0.66 


=  3-04 


Or  

Norm  of 
8. 

7. 
06) 

An  

.    13- 

>•  2  1  .  4O 
34  S 

1 

Lc    

.    25. 

Cl  ) 

•53-73 

Ne      .... 

6. 

82  $32-  33 

} 

Di  

OI  

4. 

04  ) 

Mt  

4. 

4I  ) 

11  

2  . 

74       7'IS 

45-21 

AD 

I  . 

OI           I  .  OI 

Rest  

98. 
I  . 

94 

32 

Class  

Ratios  of  I. 
Sal 

Order  .... 

Fern 
F 

"L 

CaO' 
K2O' 

Na2O' 

100.17 


This  analysis  differs  from  all  those  previously  given  in  the  lower  alumina  and 
higher  lime,  this  change  marking  the  position  of  the  rock  in  the  salfemane  class. 
As  regards  the  magmatic  position  the  type  is  well  toward  the  center  of  the  various 
divisions,  except  in  that  of  rang,  where  it  stands  almost  exactly  on  the  border  between 
rangs  7  and  8,  so  that  the  subrang  is  transitional  between  jugose  (III.  7.  2.  2),  where 
it  belongs  strictly,  and  albanose  (III.  8.  2.  2). 

Analysis  II  resembles  I  rather  closely  in  silica,  alumina,  magnesia,  soda,  and 
phosphoric  oxide,  but  differs  to  a  very  marked  extent  in  the  oxides  of  iron,  espe- 


PETROGRAPHY. 


125 


dally  ferrous,  lime,  and  potash.  That  the  last  is  very  much  too  low  is  clear  from 
the  fact  that  1.73  per  cent  of  potash  would  furnish  but  7.85  per  cent  of  leucite, 
a  figure  which  is  only  about  one-quarter  of  the  amount  of  this  mineral  actually 
present.  The  norm  of  analysis  II  would  place  the  rock  in  auvergnose  (with 
gabbros,  diabases,  and  feldspar-basalts),  no  leucitic  types  of  which  have  ever  been 
described  or  probably  could  exist,  and,  furthermore,  all  rocks  with  high  (about 
50  per  cent)  feldspar,  especially  plagioclase,  which  is  almost  wanting  in  the  Monte 
Jugo  rock,  as  we  have  seen.  This  analysis  of  Ricciardi's  must  then  be  rejected, 
as  well  as  many  others  of  similar  rocks  published  in  the  above-cited  paper  by 
Klein,  study  of  the  norms  and  the  possible  mineral  compositions  showing  them  to 
be  quite  unreliable. 

Mode. — The  mode  was  calculated  from  the  norm,  the  figures  being  checked  so 
far  as  possible  by  study  of  sections  under  the  microscope.  In  this  calculation  all  the 
potash  was  assumed  to  go  into  leucite  and  the  augite  to  have  the  usual  composition. 

Labradorite,  Ab2An3 10. 5 

Leucite 32 . 3 

Nephelite 3.4 

Augite 47.  i 

Olivine 4.0 

Magnetite i .  i 

Apatite i .  o 

JOO.O 

Occurrence. — This  type  is  confined,  apparently,  to  the  Vulsinian  District,  the 
type  specimen  coming  from  the  small  parasitic  cone  of  Monte  Jugo,  in  the  south- 
eastern part.  Other  specimens  which  may  be  referred  here  are  from  the  quarry  of 
Le  Grazie  near  Montefiascone,  from  La  Canonica  near  Orvieto,  and  from  blocks 
southwest  of  Valentino. 

Name. — The  name  of  the  subrang  III.  7.  2.  2  is,  of  course,  derived  from  that 
of  the  small  cone  from  which  comes  the  type  specimen.  The  type  rock  from  this 
locality  is  called  by  Klein*  "  a  leucite-tephrite,  passing  into  leucitite."  In  view  of 
the  very  subordinate  amount  of  feldspar  it  would  seem  that  the  name  leucitite 
would  be  appropriate  in  prevailing  classifications. 

QALERAL  ALBANOSE-JUQOSE.    II.  8-7.  2.  2, 

Megascopic  characters. — Very  dark  gray  or  black,  compact,  aphyric,  or  with  very  rare 
phenocrysts  of  leucite  and  augite.  Groundmass  aphanitic. 

Microscopic  characters. — Holocrystalline,  microporphyritic,  somewhat  clathrate  fabric. 
Microphenocrysts:  about  45  per  cent,  leucite,  augite.  Microgroundmass :  about  55  per  cent, 
augite,  labradorite,  olivine,  nephelite,  magnetite,  apatite. 

Labradorite,  Ab2An3. — Groundmass:  about  10  per  cent,  anhedral,  as  formless  interstitial 
areas. 

Leucite. — Microphenocrysts:  about  30  per  cent,  o.io  to  0.20  mm.,  anhedral,  equant, 
circular  sections,  inclusions  few. 

Nephelite. — Groundmass:  less  than  5  per  cent,  anhedral,  formless,  interstitial  areas,  diffi- 
cult to  detect. 

*  Klein,  Neu.  Jahrb.,  B.  B.  VI,  1889,  p.  20. 


126  THE  ROMAN  COMAGMATIC   REGION. 

Augite. — Microphenocrysts:  about  15  per  cent,  o.  10  to  0.50  mm.,  anhedral  to  subhedral, 
prismatic  and  fragmentary,  pale  greenish-gray,  often  zonal,  inclusions  few.  Groundmass: 
about  30  per  cent,  0.02  to  o.iomm.,  anhedral,  prismatic  to  equant,  very  pale  greenish-gray, 
arrangement  somewhat  tangential  around  the  leucites. 

Olivine. — Groundmass:  about  4  per  cent,  0.05  to  o.  10,  anhedral,  equant,  colorless. 

Magnetite. — About  2  per  cent,  o.oi  to  0.05  mm.,  anhedral,  equant. 

Apatite. — About  i  per  cent,  0.02  to  0.05  mm.,  subhedral,  prismatic. 

Chemical  composition  and  norm  as  on  p.  124. 

Type  specimen  from  Monte  Jugo,  Vulsinian  District. 

HI.  7.  3.  2.    Fiordinal  Fiasconose  [Leucite-Basanite,  Fiordine  Type]. 

Megascopic  characters. — This  type  is  very  dark  gray  and  porphyritic.  Abun- 
dant phenocrysts  of  dark-green  or  black  augite,  from  2  to  10  and  even  20  mm. 
long,  with  fewer  but  still  numerous  phenocrysts  of  olive-green  olivine,  of  about  the 
same  sizes,  constitute  some  30  per  cent  of  the  rock  volume.  The  groundmass  in 
which  they  lie  is  dark  gray,  compact,  and  quite  aphanitic.  The  rock  is  dense  and 
heavy,  evidently  highly  femic,  and  in  the  field  would  go  under  the  name  of  olivine- 
augite  melaphyre. 

Microscopic  characters. — The  very  abundant  augite  phenocrysts  are  for  the  most 
part  euhedral  to  subhedral,  often  "corroded"  at  the  borders  into  small  pits  and 
pockets,  and  not  uncommonly  fragmentary.  They  are  of  the  usual  very  pale-gray 
color,  here  without  any  tinge  of  green,  and  are  remarkably  free  from  inclusions, 
small  grains  of  magnetite  being  the  only  ones  observed.  The  large  olivines  are 
similarly  euhedral  to  subhedral,  often  fragmentary,  showing  the  common  simple 
planes,  quite  colorless,  as  free  from  inclusions  as  the  augites  (here  also  magnetite 
being  the  only  included  mineral),  and  are  quite  fresh,  except  for  a  thin  external  dark 
brown  zone. 

The  groundmass  in  which  these  lie  contains  many  small,  equant  anhedra  of 
augite  and  euhedral  crystals  of  olivine  which  run  serially  up  to  the  phenocrysts  in 
size.  With  them  are  considerable  numbers  of  very  small,  round,  water-clear  leu- 
cites,  in  part  subhedral  with  crystal  planes  and  in  part  completely  anhedral,  the 
double  refraction  being  weak  but  discernible  with  a  selenite  plate.  There  are  also  a 
few  small,  irregular  areas  of  brownish  biotite  and  some  magnetite  grains,  but  no 
feldspar  laths  or  apatite. 

The  colorless  base  between  these  crystals  shows  considerable  diversity  in  dif- 
ferent parts  of  the  section.  In  places  it  shows  the  birefringence  and  twinning 
lamellae  of  plagioclase,  the  extinction  angles  of  which  correspond  to  an  almost  pure 
anorthite.  These  feldspars  form  irregular  areas  inclosing  poikilitically  the  smaller 
crystals,  and  with  the  small  leucites  standing  out  well  against  them  on  account  of 
the  considerably  lower  refractive  index.  Elsewhere  the  patches  are  more  feebly 
biref  ringent  and  without  twinning  lamellae,  and  are  regarded  as  nephelite,  while  again 
the  base  is  isotropic,  and  is  either  nephelite  in  basal  section  or  less  commonly  glass. 

Chemical  composition. — An  analysis  of  the  rock  from  near  Fiordine  was  made, 
a  large  amount  of  material  being  taken  to  obtain  a  representative  sample.  For  com- 


PETROGRAPHY. 


127 


parison  is  also  given  the  only  other  known  analysis  of  a  rock  of  the  same  subrang, 
a  "ouachitite"  from  a  dike  near  Hot  Springs  in  Arkansas.  According  to  Kemp's 
description*  this  is  highly  biotitic  and  is  to  be  regarded  as  a  distinct  type,  to  which 
the  name  of  ouachital  fiasconose  may  be  given.  An  analysis  of  a  rock  from  Mon- 
tana belonging  to  a  closely  related  magma,  described  by  Pirsson,  to  which  the  type 
name  of  arrowal  cascadose  may  be  given,  will  be  found  in  III. 

Chemical  Composition  of  Fiordinal  Fiasconose  [Leucite-basanite]. 


I. 

II. 

III. 

SiO  i  

44.89     o 

748 

36.4.O 

A12O3  

12  .  73 

i2<; 

12    O4 

Fe,O, 

•J  .  ?! 

02  1 

8    27 

**•  2J 

7  86 

FeO  

4-  3S 

061 

4.  en 

MeO.  . 

17.  71 

•343 

II  .44 

TO    *8 

CaO  

12.85 

231 

14.46 

8   07 

Na2O  

1  .02 

OI7 

O.Q7 

KaO    

3.66 

O3Q 

3.OI 

577 

H2O  +    

i  .  <;Q 

) 

H2O  —  

O.  27 

5      2.36 

2.8; 

CO2  

none 

3-04 

TiO2  

O.QS 

OI2 

0.42 

O.64 

ZrO2   

trace 

P2OS  

O.23 

OO2 

1.04 

I.  14 

SO3  

none 

trace 

Cl  

O.  II 

Cr2O3  

O.O3 

MnO  

n.d. 

n.d. 

trace 

BaO  

0.08 

0.48 

SrO  

O.2S 

99-77 

99.84 

99.76 

I.  Fiordinal  fiasconose  (leucite-basanite).     Fiordine,  near  Montefiascone,  Vulsinian  District. 

Washington,  analyst. 
II.  Ouachital  fiasconose  (ouachitite).   Dike  18,  near  Hot  Springs,  Arkansas.    Eakins,  analyst. 

J.  F.  Williams,  Ann.  Rep.  Geol.  Surv.  Ark.,  1890,  II,  p.  399. 

III.  Arrowal  cascadose  (minette).     Dike  near  Arrow   Peak,   Highwood  Mountains,  Montana. 
Foote,  analyst.     L.  V.  Pirsson,  Bull.  U.  S.  G.  S.  No.  237,  1905,  p.  145. 

Ratios  of  7. 

Sal 
Class 

41.14 


Or        

Norm  of  I. 
.  o.e.6) 

An  

19.18) 

19-74, 

LC  

.  .16.  e,7  ) 

Ne  

.   4.83  \ 

21.40) 

Di    

..33.86) 

Ol  

re.  63  S 

49-49 

Mt  

•   4-87) 

11  

.    i  .  82  \ 

6.69 

Ap.. 

.      O.6?. 

0.63 

Rest  

97-95 
i  .  86 

Fern 


=  o.  72 


Order , 


56.81 


Rang. 


F 
'L 

K2O'+Na2O' 


CaO' 


Subrang 


K2O' 

'Na2O' 


'0.92 


=  0.81 


•2.29 


99.81 


The  analysis  shown  in  I  is  remarkable  as  being  the  lowest  in  silica,  alumina, 
and  potash,  and  the  highest  in  magnesia  and  lime,  of  any  of  the  rocks  analyzed.   As 

*  Kemp,  In  Williams,  Ann.  Rep.  Geol.  Surv.  Ark.,  II,  1890,  pp.  393  ff. 


128  THE  ROMAN  COMAGMATIC  REGION. 

far  as  the  norm  goes,  it  is  seen  to  be  well  within  all  the  divisions,  and  this,  as  well 
the  normative  mode  of  the  type,  its  fresh  and  unaltered  condition,  caused  its  locality 
to  be  chosen  for  the  name  of  the  subrang  rather  than  the  locality  root  of  the  longer- 
known  ouachitite.  The  chemical  resemblance  to  the  Arkansas  rock  is  very  close  on 
the  whole,  though  there  are  some  differences  of  importance,  notably  in  the  silica, 
ferric  oxide,  lime,  and  magnesia.  As  this  last  rock  is  far  from  fresh,  some  of  these 
differences  may  be  ascribed  to  alteration,  and  its  analysis  is  not  very  significant. 
The  Montana  rock,  while  closely  similar  to  the  Italian  in  silica,  alumina,  and  iron 
oxides,  differs  notably  in  the  figures  for  magnesia,  lime,  and  the  two  alkalis,  the  lime 
being  very  considerably  lower  and  the  others  higher  than  in  I.  The  differences  are 
expressed  in  its  position,  III.  7.  2.  3.  It  is  of  interest  to  note  that  of  these  closely 
related  magmas,  the  fiordinal  type,  a  lava  flow,  is  almost  strictly  normative,  the 
leucites  shown  by  the  norm  appearing  modally  as  such,  while  in  the  other  two  types, 
which  occur  as  dikes,  the  mode  is  abnormative,  the  normative  leucite  and  much  of 
the  normative  olivine  molecules*  combining  to  form  modal  biotite,  the  rocks  showing 
no  leucite. 

Mode. — Owing  to  the  difficulty  of  determining  the  exact  areas  of  the  dark  pheno- 
crysts  against  the  dark  groundmass  in  the  hand  specimen,  the  amount  of  these 
could  be  determined  only  approximately,  and  the  sections  were  not  large  enough 
to  allow  a  representative  area  to  be  measured  under  the  microscope.  For  these 
reasons  the  estimation  of  the  mode  by  Rosiwal's  method  was  abandoned,  though 
sufficiently  exact  data  were  obtained  to  serve  as  some  check  on  the  results  of  the  cal- 
culation of  the  mode  from  the  norm.  This  offered  no  difficulties,  all  the  potash 
going  into  leucite,  the  normative  olivine  being  accepted  as  the  modal,  and  the  augite 
being  assumed  to  have  the  usual  composition.  The  amount  of  silica  left  to  com- 
bine with  the  soda,  after  formation  of  the  other  modal  minerals,  is  barely  more  than 
is  requisite  to  form  nephelite,  the  amount  of  albite  being  thus  negligible,  which 
agrees  with  the  character  of  the  plagioclase  determined  by  the  extinction  angles  as 
an  almost  pure  anorthite.  The  very  small  amount  of  biotite  was  neglected  in  the 

calculation. 

Anorthite 13.1 

Leucite 17.3 

Nephelite 3.4 

Augite 48. o 

Olivine 15.9 

Ores 1.7 

Apatite 0.6 

100.0 

The  mode  is  thus  normative  and  the  type  would  be  described  as  a  normative 
alferfemphyro-fiasconose. 

Occurrence. — As  far  as  known  this  type  occurs  only  in  the  vicinity  of  Monte- 
fiascone,  in  the  southeastern  part  of  the  Vulsinian  District,  especially  near  Fiordine. 
Some  of  the  small  valleys  and  dry  stream  beds  here  yield  many  well-formed,  small 
crystals  of  both  augite  and  olivine,  derived  from  this  rock.  A  somewhat  similar  type 

*  The  ouachital  type  is  apparently  vitreous. 


PETROGRAPHY.  129 

is  met  with  as  a  flow  at  the  small  hill  of  San  Francesco,  southeast  of  Ceccano,  in  the 
Hernican  District.  This,  however,  is  lighter  in  color,  though  it  presents  much  the 
same,  but  smaller,  phenocrysts  of  olivine  and  augite.  Its  groundmass  is  much  finer 
grained,  with  numerous  small  prismoids  of  augite,  but  little  olivine,  magnetite 
grains,  and  no  biotite,  embedded  in  a  colorless  base.  This  last  is  seen  to  be  in  part 
of  plagioclase,  in  small,  short  laths,  and  in  part  isotropic,  and  possibly  leucite. 
Unfortunately  no  analysis  has,  as  yet,  been  made  of  this  rock,  the  specimens  of 
which  are  not  very  fresh,  so  that  its  assignment  to  this  type  is  but  provisional  and 
somewhat  doubtful. 

Name. — The  name  of  the  subrang  to  which  this  type  belongs,  III.  7.  3.  2,  is 
derived  from  Montefiascone,  the  chief  town  in  the  neighborhood,  and  the  typal 
adjective  is  derived  from  the  hamlet  of  Fiordine,  near  which  the  type  specimen 
analyzed  was  found.  •* 

The  type  from  Fiordine  has  not  yet  been  described,  it  would  appear,  but  under 
the  prevailing  classifications  there  is  no  doubt  that  it  would  be  called  a  leucite- 
basanite,  though  the  amounts  of  leucite  and  feldspar  are  not  very  great.  The  type 
from  San  Francesco,  near  Ceccano,  which  is  very  doubtfully  referred  here,  is  called 
by  Viola  a  feldspar-basalt,  though  he  also  mentions  a  leucite-basanite  from  San 
Francesco,  apparently  a  distinct  rock,  of  which  I  found  no  specimens.  In  the 
absence  of  an  analysis,  and  in  view  of  the  difficulty  as  to  deciding  whether  leucite 
is  present  or  not  in  the  fine-grained  groundmass,  the  name  feldspar-basalt  would 
seem  to  be  appropriate  for  this  last  occurrence,  which,  if  found  to  be  leucitic  and  by 
chemical  analysis  shown  to  belong  to  the  subrang  III.  7.  3.  2,  could  be  called 
ceccanal  fiasconose. 

PIORDINAL  FIASCONOSE.    III.  7.  3.  2. 

Megascopic  characters. — Dark  gray,  compact,  highly  porphyritic.  Augite  phenocrysts 
abundant,  2  to  10  mm.,  prismatic,  equant  and  irregular,  black  or  dark  green.  Olivine  pheno- 
crysts common,  2  to  20  mm.,  equant,  olive-green.  Groundmass:  dark  gray,  aphanitic. 

Microscopic  characters. — Holocrystalline,  magnophyric,  dopatic.  Phenocrysts:  about  30  per 
cent,  augite,  olivine.  Groundmass:  about  71  per  cent,  xenomorphic  granular,  augite,  leucite 
anorthite,  olivine,  biotite,  magnetite,  apatite. 

Anorthite. — Groundmass:  about  13  per  cent,  anhedral,  poikilitic  areas,  twinned,  inclosing 
leucite,  augite,  olivine,  magnetite. 

Leucite. — Groundmass:  about  17  per  cent,  0.02  to  o.  10  anhedral,  equant,  round  sections, 
clear,  free  from  inclusions,  inclosed  poikilitically  in  the  anorthite. 

Nephelite. — Groundmass:    about  3  per  cent,  anhedral,  interstitial. 

Augite. — Phenocrysts:  about  25  per  cent,  0.05  to  20.0  mm.,  euhedral  to  subhedral,  pris- 
matic, equant,  and  irregular,  often  fragmentary,  pale  gray,  few  inclusions  of  magnetite.  Ground- 
mass:  about  25  per  cent,  0.02  to  0.50  mm.,  anhedral,  equant  and  prismatic,  very  pale  gray. 

Olivine. — Phenocrysts:  about  10  per  cent,  0.5  to  20  mm.,  subhedral  to  anhedral,  equant 
and  tabular,  colorless,  only  slightly  altered  on  edges,  few  inclusions  of  magnetite.  Ground- 
mass  :  about  5  per  cent,  o .  05  to  o .  50  mm.,  euhedral  to  subhedral,  equant,  prismatic  and  tabular, 
colorless,  very  slightly  altered. 

Biotite. — Groundmass:  about  2  per  cent,  o.i  to  0.5  mm.,  anhedral,  as  irregular,  inter- 
stitial patches,  often  inclosing  augite,  olivine,  and  magnetite  poikilitically,  brown. 

Magnetite. — Groundmass:    about  2  per  cent,  o.oi  to  0.05  mm.,  anhedral,*equant. 


130  THE  ROMAN  COMAGMATIC  REGION. 

Apatite. — Groundmass:   about  i  per  cent  or  less,  0.02  to  0.05  mm.,  subhedral,  prismatic, 
difficult  to  detect. 

Chemical  composition  and  norm  as  on  p.  127. 

Type  specimen  from  Fiordine,  near  Montefiascone,  Vulsinian  District. 

III.  8.  2.  2.    Rornal  Albanose  [Leucltite,  Rome  Type]. 

Megascopic  characters. — The  rocks  which  belong  to  this  type  are  very  uniform 
in  character  megascopically,  dense,  dark-gray  or  black  basalts,  wholly  aphyric  or 
almost  wholly  so,  the  phenocrysts  being  very  rare,  small  leucites  and  augites. 
The  texture  is  aphanitic,  no  distinction  between  dark  and  light  mineral  particles 
being  visible  to  the  naked  eye  or  with  the  hand  lens. 

Microscopic  characters. — In  thin  section  this  type  presents  much  the  same 
features  as  those  described  under  galeral  braccianose,  except  that  the  small  amount 
of  feldspar  visible  in  that  is  here  totally  absent  or  almost  so.  The  texture  is  holo- 
crystalline,  and  the  fabric  a  more  or  less  clathrate  one,  with  the  characteristic  round 
leucites,  though  transitions  to  the  xenomorphic  granular  fabric  of  the  hernical  and 
saccal  types  are  not  rare.  These  are  somewhat  difficult  to  classify  with  certainty, 
but  the  general  rule  that  microphenocrysts  of  leucite  characterize  the  galeral  andromal 
types,  and  microphenocrysts  of  augite  the  hernical  and  saccal,  is  a  useful  diagnostic 
character.  The  small  (0.05  to  0.2  mm.)  round  leucites  are  abundant,  but  relatively 
not  as  much  so  as  in  the  corresponding  type  of  braccianose  or  vesuvose.  Skeleton 
development  is  rather  common.  Augite  microphenocrysts  are  not  abundant,  and 
assume  the  form  of  stout  subhedral  or  anhedral  prismoids,  of  the  usual  color  and 
optical  characters.  The  greater  part  of  this  mineral  is  in  the  form  of  minute  prisms 
and  grains  between  the  leucites,  as  already  described.  Magnetite  grains  are  always 
present,  in  varying  but  small  amounts,  and  the  same  is  true  of  apatite.  There  is  a 
small  quantity  of  colorless,  interstitial  base,  the  exact  study  of  which  is  difficult, 
owing  to  the  felted  character  of  the  microgroundmass.  In  places  it  appears  to  be 
plagioclase,  again  nephelite  or  glass,  though  these  are  rarer,  but  the  amount  in  any 
case  is  small.  In  some  specimens  there  are  small  patches  of  yellowish,  interstitial 
melilite,  marking  a  transition  to  the  boval  type,  but  in  typical  ones  this  is  negligible 
or  wholly  absent.  In  some  instances  there  may  be  a  trifling  amount  of  interstitial 
brownish  biotite.  Olivine  is  sometimes  present,  in  rare  small  grains,  and  again 
wholly  absent. 

Chemical  composition. — An  analysis  of  a  characteristic  specimen  of  this  type 
was  made  and  is  given  below  in  I.  There  is  also  given  for  comparison  in  II  an  analy- 
sis of  the  coarse  granular  type  from  the  High  wood  Mountains  (missourite),  which 
is  magmaticaUy  very  close  to  the  dofemane  class,  where  it  was  formerly  placed,*  but 
which  the  more  accurately  calculated  norm  of  Pirssonf  has  shown  to  be  just  across 
the  border  in  salfemane,  and  hence  in  albanose.  To  this  type  the  name  "  missoural 
albanose  "  may  be  applied,  in  recognition  of  its  name  in  the  prevailing  systems. 

*  Washington,  Prof.  Paper,  U.  S.  Geol.  Surv.  No.  14,  1003,  p.  355. 
t  Pirsson,  Bull.  U.  S.  Geol.  Surv.  No.  237,  1905,  p.  120. 


PETROGRAPHY. 

Chemical  Composition  of  Romal  Albanose  {Leucitite}. 


I. 

II. 

I. 

II. 

SiO2     

4.6.24     O.77I 

46.06 

COj  

none 

A1,O, 

14..  42          .  141 

IO.OI 

TiOa  

1  .  17      O.  1C 

O.  11 

Fe,O» 

4.  06          .  O2? 

•i.  17 

ZrOj  

trace 

FeO  

4.36          .061 

5.61 

P2Oc.. 

0.41       .003 

O.  21 

MgO  .  . 

6.OQ          -HT( 

14.  74 

SO,  .. 

O.O2 

O.O? 

CaO  

1^.24         .237 

10.  cc 

Cl  

O.O1 

Na2O  

i.6<;       .027 

i  •  31 

MnO  

n.d. 

trace 

KaO  

6.17       .068 

c.  14 

BaO  

O.  11 

O.  12 

H,O  +  '    .  . 

0.78 

1  .44 

SrO  

o  20 

Hn 

°-57 

100.41 

99-57 

I.  Romal  albanose  [leucitite].     Upper  flow  of  Monte  Rado,  east  of  Lake  Bolsena,  Vulsinian 

District.     Washington,  analyst. 

II.  Missoural  albanose  [missourite].     Shonkin  Creek,  Highwood  Mountains,  Montana.   Hurl- 
but,  analyst.     Pirsson,  Bull.  U.  S.  G.  S.  No.  237,  1905,  p.  117. 


Norm  of  I. 

An  .................    12-79 

Lc  ..................   29.65 

Ne  .................     7.67 


Ratios  of  I. 


I2-79 


oi  

2  .6?  r  IQ 

77 

Am  

2  .  O2  ) 

Mt  

5.80  )    o 

n  

2.28? 

08 

Ap.. 

O.O7         O 

07 

Rest  

98-93 
i  .50 

50.11 


48.82 


Class 


Sal 
'Fern 


Order 


Rang. . . 
Subrang 


CaO' 
KaO' 


'  Na2O' 


=  0.34 


•2.07 


=  2.52 


100.43 


The  analysis  in  I  resembles  in  general  the  preceding  ones  of  types  of  brac- 
cianose,  which  have  also  been  called  leucitite,  or  leucite-tephrite,  but  differs  in  the 
somewhat  lower  alumina  and  potash,  and  in  the  higher  lime.  Comparison  with  the 
missoural  type  shows  the  general  resemblance  and  at  the  same  time  the  much  more 
femic  character  of  the  latter  (the  norm  of  which  is  published  by  Pirsson  in  the  paper 
cited).  This  difference  is  brought  out  by  comparison  of  the  ratios,  especially  those 
for  class,  which  in  I  is  almost  exactly  that  of  the  center  point,  while  in  II  it  is  almost 
exactly  at  the  border  between  Classes  I  and  II.  Otherwise  both  are  well  within  the 
other  divisions,  the  ratios  of  the  alkalis  being  almost  identical. 

Mode. — For  the  same  reasons  that  have  obtained  before  it  was  impracticable 
to  measure  the  mode  of  this  type  optically.  It  was  calculated  from  the  norm  as 
usual,  the  only  new  assumption  being  that  the  small  amount  of  melilite,  known  to  be 
present  by  microscopical  study  and  indicated  by  the  norm,  had  the  composition 
of  that  of  Monte  Somma  as  analyzed  by  Bodlander.*  In  II  is  given  the  mode  of  mis- 
soural albanose  [missourite]  calculated  by  Pirsson. f 

*  Bodlander,  Neu.  Jahrb.  I,  1893,  p.  17. 
t  Pirsson,  loc.  cit.,  p.  1 18. 


132  THE  ROMAN  COMAGMATIC  REGION. 

I.  n. 

Anorthite 5.3  none 

Leucite 30.0  16 

Nephelite 5.8  none 

Analcite none  4 

Zeolites none  4 

Augite 50.4  50 

Olivine 2.3  15 

Melilite 3.4  none 

Biotite trace  6 

Ores 1.8  5 

Apatite i .  o  .... 

IOO.O  100 

The  figures  of  I  correspond  with  the  appearance  in  thin  section,  and  may  be 
regarded  as  approximately  correct.  As  compared  wth  the  missoural  type  the  amount 
of  augite  in  each  is  the  same,  and  the  nephelite  of  I  is  replaced  by  analcite  and  zeolite 
in  II.  The  chief  differences  are  found  in  the  much  higher  leucite  and  lower  olivine 
of  I,  as  well  as  in  the  presence  in  this  of  anorthite  and  melilite  and  in  II  of  biotite, 
all  in  small  amounts.  It  is  clear  that  the  mode  of  I  is  practically  normative,  the  re- 
adjustments needed  to  form  augite  and  melilite  being  negligible,  while  the  mode  of  II 
is  more  abnormative,  about  one-quarter  of  the  normative  leucite  going  into  biotite.* 
The  romal  type  may  then  be  described  as  normative  salphyri-albanose,  while  the 
missoural  type  would  be  a  biotitic  grano-albanose. 

Occurrence. — This  type  is  abundant  in  certain  districts,  especially  in  the  Vul- 
sinian,  the  Sabatinian,  and  the  Latian.  In  the  first  some  localities  are  near  Sa 
Lorenzo  and  at  the  cemetery  of  Bolsena,  at  the  upper  flow  in  the  quarry  on  the  west 
side  of  Monte  Rado,  and  at  the  Ponte  della  Regina  and  Madonna  del  Borgale,  south- 
east of  Lake  Bolsena.  In  the  Sabatinian  District  the  most  prominent  locality  is 
the  crater  of  Trevignano,  on  the  north  shore  of  Lake  Bracciano.  Most  of  the  exten- 
sive flows  south  of  the  lake  with  a  similar  habit  belong  to  braccianose,  as  we  have 
seen,  but  at  Casaccia,  near  the  Anguillara  station,  this  type  of  albanose  occurs.  In 
the  Latian  District  the  type  is  common,  and  some  localities  are  Monte  Cavo,  Grotta 
Ferrata,  and  the  Villa  Aldobrandini,  near  Frascati. 

Name. — The  name  of  the  subrang  is  derived  from  that  of  the  Alban  Hills,  in 
which  this  magma  is  represented  abundantly.  That  of  the  type  is  derived  from  that 
of  the  region,  of  which  this  is  one  of  the  most  characteristic  types. 

In  the  prevailing  systems  these  rocks  have  been  uniformly  called  leucitites, 
and  this  may  be  regarded  as  their  proper  name,  though  the  term  should  logically,  it 
would  seem,  be  reserved  for  rocks  composed  wholly  of  leucite  or  almost  so.f 

ROMAL  ALBANOSE.    Ill,  8.  2.  2. 

Megascopic  characters. — Very  dark  gray  or  black,  compact,  aphyric,  or  with  very  rare 
phenocrysts  of  leucite  and  augite.  Groundmass  aphanitic. 

*  As  4.0  per  cent  of  the  normative  leucite  is  present  in  the  biotite  it  would  seem  that  the  modal  amoun 
of  biotite  is  really  somewhat  more  than  is  here  calculated. 

t  Cf.  Cross,  Am.  Jour.  Sci.,  IV,  1897,  p.  137;    and  Washington,  ibid.,  IX,  1000,  p.  54. 


PETROGRAPHY.  133 

Microscopic  characters. — Holocrystalline,  microporphyritic,  clathrate  microfabric.  Micro- 
phenocrysts:  about  50  per  cent,  leucite,  augite.  Microgroundmass:  about  60  per  cent,  augite, 
anorthite,  olivine,  magnetite,  apatite,  sometimes  melilite  and  biotite. 

Anorthite. — Groundmass:    about  5  per  cent,  anhedral,  interstitial,  difficult  to  distinguish. 

Leucite. — Microphenocrysts:  about  30  per  cent,  0.05  to  o.  20  mm.,  subhedral  to  anhedral, 
equant,  round  sections,  sometimes  skeleton  forms,  inclusions  common,  very  small,  mostly  of 
glass  or  augite,  regularly  arranged. 

Nephelite. — Groundmass:   about  5  per  cent,  anhedral,  interstitial  areas,  difficult  to  detect. 

Augite. — Microphenocrysts:  about  20  per  cent,  o.  10  to  0.30  mm.,  anhedral,  prismatic,  or 
fragmentary,  pale  yellowish-gray,  inclusions  few.  Microgroundmass:  about  30  per  cent,  0.02 
to  0.05  mm.,  anhedral,  prismatic  to  equant,  very  pale  yellowish-gray,  arrangement  partly 
tangential  around  the  leucites. 

Olivine. — Groundmass:  2  to  o  per  cent,  0.05  to  o.  10  mm.,  anhedral,  equant. 

Biotite. — i  to  o  per  cent,  anhedral,  small  interstitial  areas. 

Melilite. — Groundmass:  3  to  o  per  cent,  anhedral,  interstitial  areas. 

Magnetite. — Groundmass:    about  2  per  cent,  o.oi  to  0.05  mm.,  anhedral,  equant. 

Apatite. — About  i  per  cent,  0.03  to  0.05  mm.,  subhedral,  prismatic. 

Chemical  composition  and  norm  as  on  p.  131. 

Type  specimen  from  Monte  Rado,  east  of  Lake  Bolsena,  Vulsinian  District. 

III.  8.  2.  2.    Saccal  Albanose  [Leucitite,  Sacco  Type]. 

Megascopic  characters. — In  the  hand  specimen  this  type  resembles  closely  that 
just  described,  being  dense  and  very  dark  gray,  and  almost  wholly  aphyric,  the  very 
rare  phenocrysts,  when  present,  being  of  leucite  and  augite.  In  examining  a  series 
of  specimens  of  the  two,  however,  it  is  noticed  that  the  color  of  the  romal  type  is 
distinctly  darker  as  a  rule  than  that  of  the  saccal,  though  both  are  so  dark  and 
aphanitic  as  to  be  called  basalts  in  the  field. 

Microscopic  characters. — To  this  type  the  description  of  the  hernical  braccianose 
applies  very  well,  except  as  to  the  feldspar,  which  is  here  scarcely  visible.  The  rock 
is  holocrystalline  and  with  the  same  xenomorphic  granular  fabric.  There  are  fairly 
numerous  microphenocrysts  of  augite,  from  0.2  to  0.5  mm.  long,  in  stout,  subhedral 
prismoids  and  equant  anhedral  grains.  They  are  of  the  usual  pale-gray  color, 
in  some  specimens  with  a  zonal  structure,  slightly  darker  and  greenish  toward  the 
center,  and  almost  wholly  free  from  inclusions.  This  augite  has  been  studied  by 
Viola,*  though  the  variety  examined  by  him  is  decidedly  greener  and  apparently 
richer  in  the  aegirite  molecule  than  that  shown  in  my  sections.  He  gives  the  extinc- 
tion angle  c  At  in  the  small  non-pleochroic  crystals  as  45°,  while  I  obtained  42°  for 
a  A  a.  To  this  pyroxene  he  gives  the  name  of  fedorowite.  As  an  analysis  of  typical 
augite  from  the  rocks  of  the  Roman  Region  was  desirable,  a  specimen  of  this 
type  was  chosen  for  examination  from  near  Ticchiena,  north  of  Frosinone,  in  the 
Hernican  District.  The  rock  powder  used  for  the  chemical  analysis  of  the  rock  was 
digested  for  some  hours  in  warm  dilute  hydrochloric  acid,  to  which  a  little  hydro- 
fluoric acid  was  added.  The  residue  was  washed  out  with  dilute  solution  of  sodium 
carbonate  and  the  process  repeated,  but  without  hydrofluoric  acid,  the  final  residue 
being,  of  course,  washed  perfectly  free  from  all  soluble  salts.  This  process  would 

*  C.  Viola,  Nett.  Jahrb.,  1899,  I,  pp.  102-31. 


134  THE  ROMAN  COMAGMATIC  REGION. 

naturally  dissolve  all  the  leucite,  olivine,  melilite,  magnetite,  apatite,  and  anorthite, 
which  are  the  only  other  minerals  present,  and  would  have  only  a  slight  effect  on  the 
augite,  removing  the  outer  portions.  A  few  larger  augites  in  the  specimen  were 
zonal,  but  the  majority  of  them  and  all  the  small  ones  were  quite  uniform  in  char- 
acter from  center  to  periphery.  The  analysis  resulted  as  follows: 

Analysis  of  Augite  (Fedorowite)  from  Ticchiena,  Hernican  District. 

SiOj 45 . 46  o.  758 

TiO2 2.85  .036 

A12O3 6.47  -063 

Fe2O3 6.62  .041 

FeO 5 . 64  .078 

MnO 0.15  0.002 

MgO 12 .  72  . 293 

CaO 20.17  .360 

Na20 0.72  .012 

K2O o .  65  .007 

100.45 

Combining  these  into  the  respective  amounts  of  pyroxene  molecules,  we  get  tl 
following  percentage  composition : 

(Mg,  Fe)(Al,  Fe)2SiO6   21 .00 

CaMgSiaOe 62 . 21 

CaSiO3 8.35 

(Na,  K)FeSi,O6 9.00 

100.56 

That  is,  the  pyroxene  has  the  molecular  composition  85  (Mg,  FeAl,  Fe)a 
SiO6.288CaMgSi2O6.72CaSiaO3.19(Na,  K)  FeSi2O6,  or  more  roughly  4  (Ca,Mg) 
SiO3  +  (Mg,  Fe)  (Al,  Fe)2Si2C>6  with  less  than  1:20  of  the  acmite  molecule.  Cal- 
culating the  norm  of  this  pyroxene  as  if  it  were  a  rock,  we  get: 


Wo 2.67 

Mt 9.51 

H 5-47 


Or 2.50 

An 12 . 23 

Lc i .  09 

Me 3.41 

Di 63.29  100.17 

It  is  thus  seen  that  salic  constituents  make  up  about  one-fifth  of  the  augite,  and 
may  thus  introduce  very  notable  differences  between  the  norm  and  mode  in  many 
cases. 

It  may  be  mentioned,  by  the  way,  that  the  norm  calculated  above  was  used  in 
readjusting  the  figures  of  the  norm  H  to  calculate  the  modes,  as  this  procedure  was 
found  to  be  more  easy  and  simple  than  that  of  ratios  of  the  different  chemical  con- 
stituents as  compared  with  the  data  in  the  assumed  mineral  composition,  as  ex- 
plained elsewhere.* 

Between  the  larger  augites,  and  either  between  or  inclosing  the  smaller  ones,  are 
small  formless  or  sometimes  rounded  individuals  of  leucite,  which  acts  as  an  intersti- 
tial cement  to  a  large  extent,  though  the  individuals  are  small  and  not  poikilitic  in 
the  usual  sense  of  the  term.  With  these  leucites  are  small  areas  of  melilite  in  most 

*  Cross,  Iddings,  Pirsson,  Washington,  Quant.  Class.  Ign.  Rocks,  Chicago,  1903,  pp.  216  ff. 


PETROGRAPHY. 


135 


of  the  specimens,  the  amount  of  this  varying  somewhat  and  never  being  large.  There 
is  also  a  very  little  soda-lime  feldspar,  in  minute,  stout  prismoids,  generally  a  few 
grains  of  olivine,  and  small  grains  of  magnetite  and  needles  of  apatite. 

Chemical  composition. — Two  analyses  were  made  of  rocks  of  this  type,  both 
specimens  coming  from  the  Hernican  District.  With  them  are  given  two  other 
analyses  of  rocks  from  this  area,  though  they  are  not  altogether  worthy  of  confidence. 

Chemical  Composition  of  Saccal  Albanose  [  Leucitite]. 


I. 

II. 

III. 

IV. 

SiO  2    

46  .27       O 

771 

4.7.  0?      O 

784 

4.7    CO 

4O  .  23 

A13O3    

17.  2< 

169 

17.08 

167 

18  02 

I7.OO 

Fe,O, 

3  .04 

02  C 

3OA. 

OIO 

6  87 

FeO  

1-  23 

OJ..1 

4  80 

068 

I  .  IO 

2.  O3 

MgO  .  . 

C    08 

127 

e.67 

142 

2  .  41 

4.  2O 

CaO     

II    4O 

2O4 

IO.  6l 

189 

ii  66 

12.  73 

NaaO       

2.  6? 

OAA. 

i  61 

O26 

1.84 

2.0? 

KaO  

8.c8 

091 

7.  £12 

080 

IO.OC 

<M8 

H2O  +  

O.  77 

•   *     1 

O.  71  1 

HaO-  

O.  13 

U./l     [ 

O.  22  ) 

0.72 

0.80 

COa     

none 

none 

TiO2  

i  ii 

OI4 

I.  IS 

OI4 

ZrO2  

0.03 

o.  02 

P2Oc  .  . 

o.  20 

OO2 

O   17 

OOI 

O.  Cl 

o.  30 

SO,  .  . 

trace 

none 

s  

none 

none 

CuO  

none 

O.  23 

NiO  

o  o? 

MnO  

n.d. 

n.d 

BaO  

o.  10 

O   OQ 

SrO  

0.06 

100.  61 

99.90 

100.66 

101.08 

I.  Saccal  albanose  [leucitite].     Ticchiena,  near  Frosinone,  Hernican  District.     Washington, 

analyst. 
II.  Saccal  albanose  [leucitite].      Pofi,  Hernican  District.     Washington,  analyst. 

III.  "Leucitite."      Pofi,  Hernican  District.     Speciale,  analyst.     Cf.  Viola,  loc.  cit.,  p.  100. 

IV.  "Leucite-basalt."      Morolo,   Hernican  District.     Viola,  analyst.        Viola,   Neu.  Jahrb., 

1899,  I,  p.  97. 


Norm  of  7. 


An  

.  Q.41; 

Q.4C   , 

Lc  

.  .  30  .  68  ) 

Ne  

.  12  .  CO  1 

52.18  j 

Di  

.  .  10.  II  } 

Ol  

.    3.14  > 

2O  .  ^2 

Am  

.    7.27) 

Mt  

.    S   80  ) 

11  

2    I  2   1 

7-93 

Ap  .. 

o  06 

o  96 

Rest  

IOO.O4 

61.63 


38.41 


Or  

Norm 

.     3 

7//J 

89 
96 
83 

68 
80 
96 
41; 

13. 
34 

• 
20.85^ 
39-51, 
3I-76 

6-54 
o-34 

An        .      ... 

16 

Lc  

•  •  3i 

Ne  

.    7 

Di  

.  .27 

Ol  

.      3 

Mt 

...   4 

11  

2 

Ap  .  . 

o 

Rest  

99 
i 

oo 
09 

60.36 


38.64 


100.73 


100.09 


136  THE  ROMAN  COMAGMATIC  REGION. 

Ratios. 

I.  II. 

Sal 
Class  ......................  ^  —  —  i  .  oo     1.56 

rem 

p 
Order  ......................  =-  =0.18    0.50 


Analyses  I  and  II  are  much  alike,  the  differences  being  of  small  importance. 
will  be  noticed  that  their  norms  are  unlike  in  that  the  norm  of  I  shows  considerable 
akermanite,  while  that  of  II  shows  no  akermanite,  but  a  little  orthoclase,  and  coi 
siderably  more  anorthite  and  less  leucite.  These  are  explained  by  the  slightlj 
lower  silica  and  higher  lime  and  potash  of  I.  The  two  resemble  the  analysis  of 
romal  albanose  (p.  130),  but  not  as  closely  as  they  do  each  other.  In  the  forme 
type  the  alumina  and  potash  are  considerably  lower  and  the  lime  and  magne 
higher. 

The  analyses  in  III  and  IV  are  not  very  reliable.    It  may  suffice  to  point  out 
as  very  doubtful  features,  the  high  alumina  and  low  magnesia,  the  iron  oxides,  ar 
the  high  potash,  as  well  as  the  presence  of  copper  in  III.     Copper  was  especiallj 
looked  for  in  my  analysis  of  the  Pofi  rock,  but  without  finding  even  a  trace,  and  it 
is  probable  that  the  copper  reported  by  Speciale  is  in  reality  platinum  derived  fror 
the  vessels  used.     In  IV  may  be  noted  the  somewhat  high  silica,  the  iron  oxidt 
the  high  lime,  and  the  unsatisfactory  summation.    It  may  be  noted  that  the  mag 
represented  by  this  analysis  would  fall  in  II.  6.  3.  3,*  an  improbable  position  for 
such  a  highly  leucitic  and  feldspar-poor  rock. 

Mode.  —  Neither  of  the  specimens  lent  themselves  to  microscopic  measurement, 
on  account  of  the  fineness  of  the  grain  and  the  difficulty  of  estimating  the  leucite 
and  interstitial  substance.  As  the  simpler  one  the  mode  of  the  Pofi  rock  will  be 
discussed  first.  This  calculates  out  as  follows: 

Anorthite,  AbjAn^  ..............................  14.  i 

Leucite  .........................................  35  •  ° 

Nephelite  .......................................  S  .  i 

Augite  ..........................................  41.0 

Olivine  .........................................  4.0 

Ores  ...........................................  0.5 

Apatite  .........................................  0.3 

100.0 

This  mode  corresponds  well  with  the  appearance  of  the  thin  section,  though 
the  amount  of  anorthite  is  rather  larger  than  one  would  expect.  The  adjustments 
are  of  comparatively  slight  importance,  chiefly  to  form  the  augite,  and  those  necessary 
to  change  orthoclase  to  leucite,  with  consequent  formation  of  a  little  albite  and  less 
nephelite.  The  mode  therefore  may  be  regarded  as  normative  and  the  rock 
described  as  normative  alferphyri-albanose. 

*  Washington,  Prof.  Paper,  U.  S.  Geol.  Surv.  No.  14,  1003,  p.  301. 


PETROGRAPHY.  137 

The  mode  of  the  Ticchiena  rock  was  likewise  easy  to  calculate  from  the  norm, 
but  the  results  do  not  correspond  as  well  with  the  microscopic  appearance.  This 
is  due  to  the  amount  of  normative  akermanite  present,  which  demands  the  forma- 
tion of  some  12  per  cent  of  melilite,  and  there  does  not  appear  to  be  nearly  so  much  of 
this  actually,  though  a  little  undoubtedly  exists.  There  is  no  more  normative  min- 
eral present  from  which  we  can  abstract  any  silica  to  change  the  akermanite  in 
part  to  wollastonite  to  form  augite,  and  there  is  no  reason  for  thinking  that  the 
analysis  is  defective  to  the  serious  extent  implied  by  the  normative  presence  of  so 
much  akermanite  and  so  little  modal  melilite.  As  the  grain  is  very  fine,  except  for 
the  rather  numerous  augite  microphenocrysts,  it  may  well  be  that  more  melilite  is 
actually  present  than  would  appear  in  the  section.  However  this  may  be,  the  results 
of  the  calculation  are  as  follows: 

Anorthite 3.8 

Leucite 39 . 7 

Nephelite 9.8 

Augite 28.9 

Olivine 1.6 

Melilite 12.2 

Ores 3.0 

Apatite i .  o 

100.0 

This  mode  corresponds  well  with  the  thin  section  as  far  as  the  amounts  of  an- 
orthite,  leucite,  olivine,  ores,  and  apatite  go,  and  that  of  augite  is  fairly  close,  though 
the  apparent  amount  is  somewhat  greater.  The  main  variations  from  the  thin  sec- 
tion are  in  the  higher  nephelite  and  melilite,  the  latter  of  which  has  been  spoken  of 
above.  The  mode  is  a  normative  one  on  the  whole,  but  the  implied  presence  of  so 
much  melilite,  even  though  discoverable  with  difficulty,  would  suggest  that  this 
rock  be  distinguished  from  that  of  Pofi.  This  could  be  done  by  erecting  it  into  a 
distinct  type,  whch  might  be  called  the  ticchienal,  and  distinguished  from  the  true 
saccal  by  the  presence  of  melilite,  and  from  the  boval  type  to  be  described  next, 
which  is  also  rich  in  melilite,  by  the  phenocrystic  character  of  the  augite  and  the  less 
obvious  melilite.  But  to  avoid  an  undue  number  of  type  names  this  rock  will,  for 
the  present,  be  considered  to  be  of  the  same  type  as  the  Pofi  one,  that  is,  saccal. 

Occurrence. — This  type  of  albanose  is  by  no  means  so  common  as  the  romal, 
and  specimens  could  be  identified  with  certainty  only  from  the  Hernican  District, 
at  the  two  localities  cited,  from  the  Vulsinian,  where  it  forms  the  flow  on  the  summit 
of  Monte  Rado,  above  that  mentioned  on  pp.  131  and  132,  and  in  the  Sabatinian 
at  the  Fosso  San  Celso,  on  the  south  shore  of  Lake  Bracciano. 

Name. — The  derivation  of  the  subrang  name  has  been  already  discussed.  The 
typal  adjective  is  derived  from  the  Sacco  River,  in  the  valley  of  which  rocks  of  this 
type  are  prominent. 

In  the  prevailing  classifications  these  rocks  are  for  the  most  part  to  be  regarded 
as  leucitites,  though  the  small  amount  of  plagioclase  in  some  specimens  has  suggested 
the  name  of  leucite-tephrite  as  the  more  appropriate.  The  smallness  of  the  quantity 
of  olivine  would  render  the  term  leucite-basanite  inappropriate. 


138  THE  ROMAN  COMAGMATIC  REGION. 

SACCAL  ALBANOSE.    III.  8.  2.  2. 

Megascopic  characters. — Very  dark  gray,  compact,  aphyric,  or  with  very  rare  phenocrysts 
of  augite  and  leucite,  aphanitic. 

Microscopic  characters. — Holocrystalline,  microporphyritic.  Microphenocrysts:  about  20 
per  cent,  augite.  Microgroundmass:  about  80  per  cent,  leucite,  augite,  anorthite,  nephelite. 
olivine,  magnetite,  apatite,  usually  melilite. 

Anorthite. — Four  to  10  per  cent,  anhedral,  part  as  very  small  prisms,  part  interstitial, 
sometimes  difficult  to  detect. 

Leucite. — About  35  per  cent,  0.03  to  o.iomm.,  anhedral,  equant,  usually  round  sections* 
sometimes  irregular,  clear,  inclusions  few. 

Nephelite. — About  5  per  cent,  anhedral,  interstitial  cement. 

Augite. — Microphenocrysts:  about  20  per  cent,  0.2  to  0.5  mm.,  subhedral  to  anhedral, 
prismatic,  often  fragmentary,  pale  greenish-gray,  sometimes  zonal.  Microgroundmass:  about 
30  per  cent,  0.02  to  0.2  mm.,  anhedral,  prismatic  and  equant,  very  pale  greenish-gray. 

Olivine. — 4  to  o  per  cent,  0.05   to  o.  10  mm.,  anhedral  to  subhedral,  equant,  colorless. 

Melilite. — o  to  5  per  cent,  anhedral,  as  formless,  interstitial  areas,  yellowish. 

Magnetite. — About  2  per  cent,  0.02  to  0.05  mm.,  anhedral,  equant. 

Apatite. — About  i  per  cent,  0.03  to  o.  10  mm.,  subhedral,  prismatic. 

Chemical  composition  and  norm  as  on  p.  135. 

Type  specimens  from  Pofi  and  Ticchiena,  Hernican  District. 

III.  8.  2.  2.    Boval  Albanose  [Leucitite,  Bove  Type,  Cecilite]. 

Megascopic  characters. — This  type  does  not  differ  in  the  hand  specimen,  to  an) 
marked  extent,  from  those  just  described.  The  rocks  are  dense,  very  dark  gray,  in 
this  more  like  the  hernical  type,  and  are  aphanitic.  They  are  for  the  most  part 
entirely  aphyric,  but  may  carry  very  rare  phenocrysts  of  leucite  or  augite,  as  in  the 
other  cases.  Some  of  them,  as  specimens  from  Capo  di  Bove,  show  a  peculiar,  vei 
faint  mottled  appearance,  in  patches  of  very  slightly  greenish-gray  mingled  with  the 
general  dark  gray.  This  has  been  noticed  in  other  melilite  rocks,  as  well  as  in 
more  ordinary  basalts,  and  seems  to  be  due  to  incipient  weathering. 

Microscopic  characters. — This  type  is  wholly  holocrystalline,  and  with  a  peculiar 
fabric  which  resembles  the  clathrate  described  above,  but  differs  in  the  greater 
thickness  of  the  spaces  between  the  round  leucite  spots. 

Small  (0.05  to  0.20),  round  leucites  are  abundant,  making  up  about  one-half  of 
the  rock.  Many  of  them  are  subhedral,  showing  some  crystal  planes,  while  others 
are  quite  anhedral  and  with  round  outlines.  They  very  commonly  carry  small, 
mostly  black,  inclusions,  which  are  arranged  radially  or  as  circular  lines  of  dots. 
Birefringence  is  rather  weak  but  unmistakable. 

Between  these  are  the  other  constituents.  There  is  a  good  deal  of  augite,  of 
the  usual  pale-gray  color,  which  is  largely  in  formless,  interstitial  masses,  but  occa- 
sionally subhedrally  prismatic.  This  constitutes  about  one-quarter  of  the  rock. 
Melilite  is  also  present  in  very  considerable  amount,  but  less  than  the  augite,  in 
similar  interstitial  areas,  usually  poikilitic.  The  mineral  is  of  a  very  pale  yellowish- 
gray,  with  high  relief,  shows  the  usual  blue-gray  color  between  crossed  nicols, 
and  has  often  the  well-known  "pflock"  structure.  There  are  also  similar  areas  of 


PETROGRAPHY. 


139 


clear,  colorless  nephelite,  in  about  the  same  amount  as  the  melilite,  and  rare  sub- 
hedral  prisms  of  this  mineral  are  seen  here  and  there. 

As  accessories  occur  very  small  anhedral  grains  of  olivine,  few  small,  stout  laths 
of  anorthite,  magnetite  grains,  and  apatite  needles.  In  some  specimens  there  are 
also  a  few  small  flakes  of  brownish  biotite. 

Chemical  composition. — An  analysis  was  made  of  this  type,  which  has  been 
already  published  in  incomplete  form.  In  column  II  is  given  the  only  other  analysis 
so  far  made  of  this  well-known  rock. 

Chemical  Composition  of  Boval  Albanose  [Melilitic  Leucitite,  Cecilite]. 


I. 

II. 

I. 

II. 

SiOa  

4"?.  00      0.767 

4"?.Q7 

H-O+  

0.41; 

O    SQ 

A12O3 

16.56           162 

18.72 

CO2  

none 

Fe2O3  

4.17         .026 

n.d. 

TiO2  

o.  77    0.00? 

FeO  

s.78       .071; 

10.68 

PaO<  . 

o.  =;6       .004. 

MeO 

C  .  70         .  177 

c  .67 

MnO  

n.d. 

CaO        .... 

10.47       .  188 

IO.  (!7 

BaO  

O.  2? 

NajO 

2.  l8          .O7S 

1.68 

SrO  

none 

Ko 

fi  Ri 

100.65 

100.67 

I.  Boval  albanose  [leucitite].     Capo   di   Bove,   Latian  District.     Washington,   analyst.     Cf. 

Am.  Jour.  Sci.,  IX,  1900,  p.  53. 

II.  Boval  albanose  [leucitite].     Capo  di  Bove,  Latian  District.    Bunsen,  analyst.     Roth,  Beitr. 
Petrog.,  1869,  p.  cii,  No.  31. 


Norm  of  7. 


An  

8 

62     8.62) 

Lc  

.  .41 

86  )        o    [ 

Ne  

94     Si-Bo  C 

Di  

9, 

Ol  

8 

18  '31.58 

Am  

.    7 

27 

Mt  

6. 

07         , 

11  

o 

76      6'79 

Ap.. 

i 

26            I  .  26 

Rest  

IOO 

o 

05 

7O 

100.75 


60.42 


39-63 


Class. 


Ratios  0}  7. 
Sal 


Order. 


Rang.  .  . 
Subrang , 


'Fern 

F 
'L 

K2O'-|-Na2O' 


CaO' 


K2O' 
CaO' 


=  0.17 


=  4-23 


=  2.74 


Analysis  I  resembles  those  of  the  two  preceding  types  of  albanose,  though 
it  is  decidedly  more  like  that  of  the  rock  from  Ticchiena. 

Mode. — The  sections  of  this  type  were  fairly  well  adapted  to  measurement 
under  the  microscope,  though  it  was  often  somewhat  difficult  to  distinguish  between 
the  colorless  minerals.  The  results  of  such  a  study  are  found  below.  The  mode 
was  readily  calculated,  the  usual  assumptions  being  made  as  to  the  composition  of 
the  augite  and  melilite. 


140 


THE  ROMAN  COMAGMATIC  REGION. 


CALCULATED. 

MEASURED. 

Leucite  

41  .0 

Units 
2.0^ 

Vol.%.      Sp.gr. 
=    44  .  o    X    2  .  S    = 

112  .  I 

Wt.  %. 
^8.6 

Anorthite  

3.6) 

Nephelite             .                

t 
7.  -2  \ 

460 

=    10.1    X   2.6   = 

26.3 

9.1 

Augite  .  

23.6.  j 

Olivine  

6.8  \ 

L493 

-  32-7  x  3.3  = 

107.9 

37-2 

Melilite  

12.6 

403 

=     8.8   X   2.9   = 

2C  .  C 

8.8 

Magnetite  

4.0 

1  60 

=        3.  ">     X     ^.2     = 

18.2 

6.7 

Apatite  

1  .  2 

IOO.O 

4,57i 

IOO.O 

290.O 

IOO.O 

The  correspondence  between  these  is  not  very  close,  though  they  agree  in  the 
main  features.  It  would  seem  that  overlapping  has  entered  to  a  considerable 
extent,  and  possibly  that  the  section  examined  was  not  quite  as  rich  in  melilite  as 
the  mass  of  the  rock.  On  the  whole  the  mode  is  normative,  but  the  notable  amount 
of  melilite  should  be  considered,  and  the  rock  may  therefore  be  described  as  a 
normative  melilitic  grani-albanose. 

Occurrence. — This  type  is  rather  common  in  the  Latian  District,  not  only  at 
the  well-known  locality  of  Capo  di  Bove,  but  at  Tusculum,  near  Grotta  Ferrata, 
and  elsewhere.  Rocks  which  may  be  considered  to  be  of  the  same  type  are  also 
found  at  Poggio  Selva  and  as  blocks  in  tuff  at  Madonna  d'Oro,  both  near  Monte- 
fiascone  in  the  Vulsinian  District,  and  at  the  fountain  west  of  Trevignano  in  the 
Sabatinian. 

Name. — The  type  name  is  derived  from  the  type  locality  of  Capo  di  Bove, 
between  Rome  and  the  Alban  Hills. 

In  the  prevailing  systems  this  type  has  been  uniformly  included  among  the 
leucitites,  of  which  it  is  always  cited  as  the  most  prominent  example.  In  view  of 
the  very  considerable  amount  of  melilite,  and  the  interest  due  this  mineral  on  account 
of  its  rarity,  it  might  be  well  to  revive  Cordier's*  old  name  "cecilite,"  derived  from 
the  tomb  of  Cecilia  Metella  above  the  quarry  of  Capo  di  Bove,  for  the  melilite- 
bearing  leucitites.  Such  a  distinction  would  be  advisable,  since  the  great  majority 
of  the  leucitites  are  free  from  melilite,  as  we  have  seen.  The  term  "cecilal"  can  not 
well  be  used  for  the  type  adjective  in  the  quantitative  system,  since  the  root  "cecil" 
is  already  in  use  in  the  subrang  cecilose  (V.  i1.  i2.  2). 

BOVAL  ALBANOSE.    111.  8.  2.  2. 

Megascopic  characters. — Very  dark  gray,  compact,  aphyric,  but  sometimes  with  very  rare 
phenocrysts  of  leucite  and  augite,  aphanitic.  Occasionally  slightly  mottled  if  not  quite  fresh. 

Microscopic  characters. — Holocrystalline,  granular,  partly  poikilitic,  leucite,  augite, 
melilite,  nephelite,  olivine,  anorthite,  magnetite,  apatite. 

Anorthite. — About  4  per  cent,  0.05  to  o.iomm.,  subhedral,  tabular,  twinned. 

*  Cordier,  Description  des  Roches,  Paris,  1868,  p.  117.   Cf.  C.  R.  VIII  Cong.  Geol.  Int.,  Paris,  1901,  p.  1051 

Leucite. — About  40  per  cent,  0.05  to  0.20  mm.,  subhedral  to  anhedral,  equant,  round 
sections,  clear,  inclusions  common,  small,  usually  black,  regularly  arranged,  skeleton  forms  not 
infrequent. 

Nephelite. — About  7  per  cent,  anhedral,  interstitial  areas. 


PETROGRAPHY. 


141 


Augite. — About  25  per  cent,  0.05  to  0.50  mm.,  subhedral  to  anhedral,  partly  in  prismatic 
and  equant,  partly  irregular  and  interstitial;  pale  gray,  few  inclusions. 

Olivine. — About  7  per  cent,  0.05  to  o.  10  mm.,  anhedral,  equant. 

Melilite. — About  12  per  cent,  anhedral,  interstitial  patches,  sometimes  poikilitic  about  the 
smaller  crystals,  areas  up  to  2  mm.  in  diameter,  "pflock"  structure  common,  pale  yellowish. 

Magnetite. — About  4  per  cent,  0.02  to  0.05  mm.,  anhedral,  equant. 

Apatite. — About  i  per  cent,  0.03  to  0.08  mm.,  subhedral,  prismatic. 

Chemical  composition  and  norm  as  on  p.  139. 

Type  specimen  from  Capo  di  Bove,  Latian  District. 

Correlation  of  Types. 

As  the  types  described  in  the  preceding  pages  are  numerous,  and  as  the  char- 
acteristics on  which  they  are  based  differ  in  certain  respects  from  those  which 
obtain  in  the  prevailing  systems,  it  will  be  not  amiss  to  point  out  briefly  the  chief 
features  in  which  those  of  the  quantitative  system  resemble  or  differ  from  each  other, 
and  also  to  indicate  the  correlation  between  them  and  those  of  the  prevailing  systems. 
TYPES  OF  THE  QUANTITATIVE  SYSTEM. 

In  correlating  the  types  from  the  point  of  view  of  the  quantitative  system  alone 
we  may  devote  special  attention  to  the  modal  and  textural  characters  denoted  by 
the  typal  adjective,  though  the  magmatic  character,  as  expressed  by  the  subrang 
name,  must  also  be  considered.  It  will  have  been  noticed  that  the  same  type  adjec- 
tive is  applied  to  different  subrangs.  This  has  been  the  case  when  the  subrangs 
resemble  each  other  quite  closely,  as  with  vulsinose,  pulaskose,  and  monzonose,  or 
with  vesuvose  and  albanose,  and  when  the  modal  and  textural  characters  of  the 
representatives  of  the  different  subrangs  are  so  much  alike  that  very  close  study, 
especially  from  the  chemical  side,  is  necessary  to  distinguish  between  the  rocks. 
Thus  only  a  chemical  analysis  or  careful  optical  measurements  can  distinguish 
between  the  three  types  of  bolsenal  vulsinose,  pulaskose,  and  monzonose,  the  modal 
characters  differing  but  slightly  and  the  textural  being  practically  identical.  By 
this  plan  of  a  single  type  adjective  applicable  to  several  subrangs  the  great  similarity 
between  such  types  is  made  manifest  and  the  number  of  new  terms  is  lessened. 

From  the  modal  and  textural  points  of  view  types  may  resemble  or  differ  from 
each  other  either  in  respect  to  mode,  or  to  texture,  or  to  both  together,  and  they  may 
also  form  series  of  types  with  members  which  are  homologous  modally  or  texturally. 
This  is  illustrated  by  the  following  examples: 


1.  5.  1.3. 

1.  5.  2.  2. 

II.  5.  2.  2. 

II.  5.  2.  3. 

Ischial  phlegrose. 
Cumal  phlegrose. 

Arsal  vulsinose. 
Bolsenal  vulsinose. 

Arsal  ciminose. 
Fiescolal  ciminose. 

Arsal  monzonose. 
Bolsenal  monzonose. 

II.  5.  2.  2. 

II.  6.2.  2. 

II.  7.2.2. 

Viterbal  ciminose. 
Bagnoreal  ciminose. 
(Orvietal  ciminose.) 

Foglianal  vicose. 
Bagnoreal  vicose. 
Orvietal  vicose. 

Vesbal  braccianose. 
Scalal  braccianose. 
Galeral  braccianose. 

142  THE   ROMAN  COMAGMATIC  REGION. 

In  both  sets  as  we  read  down  there  is  a  progressive  diminution  in  the  size  and 
abundance  of  the  phenocrysts,  here  assumedly  all  salic,  in  the  first  set  of  feldspar* 
and  in  the  second  of  leucite.  In  the  ischial  and  arsal  types  the  feldspar  phenocrysts 
are  large  and  numerous,  while  in  the  cumal,  bolsenal,  and  fiescolal  types  they  are 
small  and  scarce,  and  some  olivine  appears  in  the  fiescolal  type.  Similarly  in  the 
viterbal,  foglianal,  and  vesbal  types  the  leucite  phenocrysts  are  large,  abundant, 
and  prominent,  they  become  small  and  scarce  in  the  bagnoreal  and  scalal,  and 
there  are  no  megaphenocrysts  in  the  orvietal  and  galeral  types.  Types  which 
resemble  each  other  as  these  do,  which  are  found  in  the  same  row,  may  be  said  to 
be  texturally  homologous. 

An  analogous  relationship  is  manifest  modally.  Thus  the  types  of  phlegrose 
show  no  soda-lime  feldspar,  those  of  vulsinose  carry  a  very  notable  quantity  of  this, 
while  in  the  types  of  monzonose  this  mineral  becomes  more  prominent.  In  the 
leucitic  types  of  ciminose  soda-lime  feldspar  is  very  subordinate  in  amount  to  the 
alkali-feldspar,  and  there  is  no  leucite  in  the  groundmass;  in  those  of  vicose  alkali- 
feldspar  is  less  and  soda-lime  feldspar  is  much  increased,  and  there  is  much  leucite 
in  the  groundmass;  while  in  the  types  of  braccianose  alkali-feldspar  has  disappeared, 
its  place  being  taken  by  groundmass  leucite,  and  a  labradorite  or  anorthite  is  the  only 
feldspar.  Types  which  resemble  each  other  as  these  do,  which  are  found  in  the 
same  column,  may  be  said  to  be  modally  homologous. 

The  relations  are  not  always  as  simple  and  as  clearly  cut  as  in  the  above  examples, 
complications  being  introduced  by  the  presence  of  other  minerals  or  differences  in 
the  minor  textural  characters.  Thus  in  the  above  set  of  leucitic  types  there  are  a 
few  feldspar  phenocrysts  and  even  less  of  augite  present  along  with  those  of  leucite* 
and  the  fabric  of  the  groundmass,  for  instance,  may  be  a  trachytic  one  in  some  types 
and  a  felted,  granular,  or  intersertal  one  in  others.  It  will  also  frequently  happen 
that  no  homologues,  either  modal  or  textural,  are  known.  Thus  the  tavolatal  appia- 
nose,  with  its  very  large  leucite  phenocrysts  and  abundant  haiiynes,  does  not  much 
resemble  any  of  the  others,  though  it  is  texturally  similar  to  the  paglial  pulaskose 
and  some  of  the  foglianal  vicoses  which  have  similarly  huge  leucite  phenocrysts. 

Furthermore,  a  certain  degree  of  latitude  may  be  allowed  in  the  application  of 
these  homologies,  as  is  the  case  with  the  definition  of  types.  Thus  the  foglianal 
ciminose-auruncose  and  vesbal  braccianose  may  be  said  to  be  texturally  homologous 
with  the  foglianal  vicose  described  above,  though  the  size  of  the  leucite  phenocrysts 
in  these  two  types  is  smaller  than  is  true  of  the  last  in  the  type  specimens. 

CORRELATION  WITH  THE  PREVAILING  SYSTEMS. 

It  will  be  clear  from  an  examination  of  the  table  on  page  14,  as  well  as  from  a 
study  of  the  descriptions,  that  the  distinctions  between  the  types  of  the  quantitative 
classification  are  more  sharply  drawn  in  some  ways  than  is  true  of  those  of  the  pre- 
vailing systems.  Thus  rocks  which  would  be  called  vulsinite  in  the  latter  are  here 
referred  to  at  least  six  distinct  types,  being  distinguished  partly  by  their  magmatic 


PETROGRAPHY.  143 

positions  and  partly  by  their  textural  characters.  The  same  is  the  case  to  a  still  greater 
extent  with  the  leucite-tephrites,  which  are  here  referred  to  at  least  eleven  types,  while 
the  leucite-trachytes  have  been  split  up  into  six  types,  and  the  leucitites  into  seven. 

In  part  this  is  due  to  a  recognition  here  of  differences,  either  modal  or  textural, 
which  have  been  usually  disregarded  either  because  they  have  been  thought  to  be 
of  minor  importance  or  through  a  certain  reluctance  to  add  to  the  terminology  of 
petrography.  In  part  also  it  is  due  to  the  fact  that  a  very  considerable  portion 
of  the  rocks  under  investigation  are  so  extremely  scarce  and  so  rarely  met  with  out- 
side of  the  Roman  Region  that  any  recognition  of  differences  between  them  has  been 
thought  to  be  "not  worth  while."  In  regard  to  this  last  it  may  be  said  that  to  be 
logical  and  consistent  throughout  petrography  should  apply  its  principles  of  classi- 
fication and  nomenclature  impartially  and  irrespective  of  the  commonness  or  rarity 
of  the  objects  studied.  This  is  one  of  the  fundamental  principles  of  the  quantita- 
tive system  in  regard  to  the  chemical  and  mineralogical  constituents,*  and,  it  may 
be  added,  in  regard  to  the  textural  characters  as  well. 

But,  however  reluctant  one  may  be  to  increase  the  "burden  of  new  names," 
and  therefore  endeavor  to  force  clearly  distinct  types  into  old  frames,  as  by 
calling  missourite  a  leucite-gabbro;  and  however  neglectful  petrographers  may  be 
of  the  less  well-known  rocks,  and  therefore  content  to  lump  together  the  obviously 
unlike,  there  is  a  growing  recognition  of  the  need  of  increasingly  finer  distinctions, 
which  is  felt  even  with  the  rarest  types  as  the  petrographical  knowledge  of  the 
remote  portions  of  the  earth  increases. 

This  need  is  met,  not  only  by  the  frank  use  of  new  names,  but  in  part  by  ref 
erence  of  rocks  belonging  to  the  larger  groups  to  certain  "types,"  and  in  part  by  the 
use  of  qualifying  adjectives.  As  illustrations  of  these,  and  as  correlating  the  two 
systems,  it  may  be  pointed  out  that  the  ischial  phlegrose  corresponds  fairly  well  with 
Rosenbusch's  "Ponza  typus"  of  the  true  trachytes,  while  the  cumal  phlegrose  is 
approximately  the  same  as  his  "phonolitic  trachyte,"  though  there  are  textural  dis- 
tinctions made  in  the  quantitative  system  which  do  not  obtain  in  the  prevailing 
ones.  Similarly,  the  tavolatal  appianose  is  distinguished  from  the  other  leucite- 
tephrites  as  of  the  "phonolithoid  type."  Again,  the  types  of  galeral  and  hemical 
braccianose,  that  is,  leucitites  which  carry  a  little  labradorite,  are  distinguished  from 
the  romal  or  saccal  albanose  (feldspar-free  leucitites)  by  the  adjective  "tephritic." 

It  would  lead  us  too  far  astray  to  discuss  further  this  important  topic,  but  the 
general  facts  thus  briefly  pointed  out  will  serve  to  indicate  the  lines  along  which  a 
correlation  of  the  types  of  the  two  systems  may  be  studied. 

*  Cross,  Iddings,  Pirsson,  and  Washington,  op,  cit,,  p.  108. 


PETROLOGY. 
Introduction. 

Having  obtained  a  detailed  idea  of  the  petrography  of  the  region,  it  remains 
to  discuss  its  petrology,  the  relations  of  the  magmas  and  of  the  rock  types  to  each 
other,  and  the  conclusions  which  may  be  drawn  from  them.  In  this  way  we  shall 
obtain  a  knowledge  of  the  general  petrological  character  of  the  region  and  shall  be 
in  a  position  to  discuss  the  bearing  of  the  facts  on  the  theories  of  differentiation  and 
to  compare  the  region  with  others. 

These  regional  or  clan  characters  are  of  various  kinds,  of  which  the  most 
important  are  the  chemical,  the  mineralogical,  and  the  textural,  their  importance 
being  in  the  above  order,  as  the  chemical  characters  are  the  most,  and  the  textural 
the  least,  dependent  on  the  composition  of  the  magma.  With  them  should  also  be 
discussed  the  geologic  occurrence  of  the  types,  the  space  relations  or  geographic 
distribution,  the  time  relations  or  order  of  succession,  and  the  quantitative  relations 
or  relative  masses,  of  the  different  magmas  and  types,  as  well  as  the  average  com- 
position or  that  of  the  parent  magma,  whose  discussion  may  well  be  included  in 
that  of  the  closely  connected  quantitative  relations. 

Geologic  Occurrence. 

The  geologic  occurrence  of  the  rocks  has  already  been  described  in  a  previous 
chapter,  and  it  only  remains  here  to  point  out  some  of  the  general  facts  observed 
in  the  geologic  structure  of  the  volcanoes  and  their  relations  to  the  occurrence  of 
the  different  magmas. 

In  the  Roman  Region  the  occurrence  of  the  rocks  is  overwhelmingly  that  of 
lava  flows  and  beds  of  tuff,  which  have  been  ejected  at  the  surface  from  volcanic 
vents.  No  deep-seated  masses  have  been  revealed,  the  time  for  erosion  since  the 
close  of  volcanic  activity  having  been  too  short.  It  is  also  a  striking  feature  that 
dikes  are  extremely  rare,  the  most  abundant  being  in  the  inner  walls  of  Monte 
Somma,  and  the  only  other  known  occurrences  being  a  few  sporadic  cases  in  the 
Ciminian  District  observed  by  some  geologists.  It  must,  however,  be  said  that 
my  own  researches  failed  to  reveal  these,  and,  as  far  as  my  personal  knowledge  goes, 
there  is  not  a  single  dike  in  the  region  outside  the  Vesbian  Volcano. 

The  main  feature  of  interest  in  connection  with  the  geologic  occurrence  is  that 
the  more  complex  volcanic  structures  are  situated  at  or  toward  the  extremities  of 
the  zone,  and  that  they  are  at  the  same  time  much  the  most  varied,  in  their  eruptive 
rocks,  both  as  to  number  of  types  and  range  of  chemical  composition,  while  the 
simpler  volcanoes  are  toward  the  center  of  the  zone  and  show  very  few  types,  with 
a  narrow  range  in  chemical  composition.  As  far  as  the  structures  go  this  will  be 
144 


PETROLOGY.  145 

evident  from  the  geological  descriptions  of  the  different  districts,  by  which  it  is  seen 
that  the  outer  districts — the  Vulsinian,  Ciminian,  and  Sabatinian  on  the  north,  and 
the  Campanian  and  Auruncan  on  the  south — are  of  complex  volcanic  structure,  and 
for  the  most  part  made  up  of  the  products  from  several  vents,  while  of  the  inner  ones, 
the  Latian  consists  of  but  a  single  volcano,  or  rather  the  successive  sommas  and 
cones  around  a  single  vent,  and  the  Hernican  of  a  number  of  very  small  and  simple 
cones.  The  complexity  of  the  magmas  will  be  discussed  later. 

In  this  connection  it  is  interesting  to  note  that  the  volcanoes  of  the  Tuscan 
Region,  enumerated  on  p.  i,  are  all  of  a  simple  type  and  that  their  eruptive  rocks 
are  in  each  case  extremely  uniform,  the  different  types  of  each  being  chiefly  dis- 
tinguished by  their  textural  characters.*  Another  interesting  fact  in  connection 
with  these  volcanoes  is  that  dikes  are  quite  numerous,  although  they  are  so  rare 
at  the  Roman  ones. 

A  second  factor  which  may  be  significant  of  deeper  relations  is  that  the  more 
complex  volcanoes  are  very  close  to  each  other,  and  have  been  often  in  a  state  of 
activity  contemporaneously,  while  the  simpler  central  volcanoes  are  farther  apart, 
both  from  each  other  and  from  the  next  outer  complex  ones. 

As  the  occurrence  of  the  rocks  is  so  uniformly  that  of  flows  or  tuffs,  there  is 
little  opportunity  to  observe  any  relations  between  the  characters  of  the  rocks  or 
the  magmas  and  the  geologic  occurrence.  So  far  as  my  knowledge  goes,  there  is 
no  constant  or  evident  relation  between  the  two.  Magmas  of  the  most  diverse 
kinds  have  poured  forth  in  both  small  and  massive  flows  and  have  assumed  the 
most  diverse  types,  sometimes  highly  porphyritic  and  sometimes  aphyric,  some- 
times leucitic  and  sometimes  free  from  this  mineral.  The  tuffs,  likewise,  are  of 
very  diverse  petrographic  characters,  but  as  little  attention  was  paid  to  them  by 
me  no  generalization  can  be  attempted  here. 

Chemical  Characters. 

The  chemical  characters  of  the  Roman  Region  may  be  studied  in  the  annexed 
table,  where  are  collected  the  superior  analyses  which  have  been  presented  in  the 
preceding  pages,  those  heretofore  unpublished  except  in  the  collection  of  analyses 
being  indicated  by  an  asterisk.  Although  they  do  not  include  every  type  found 
in  the  region,  nor  representatives  in  each  district  of  all  the  types  which  occur  in  it, 
they  are  so  numerous  and  cover  such  a  wide  range  that  they  may  be  held  to  be 
representative  of  the  whole  and  a  satisfactory  basis  for  generalizations. 

The  chemical  characters  of  any  region  manifest  themselves  in  two  ways.  They 
may  be  constant,  or  practically  constant,  throughout  the  region,  and  consist  in  the 
prominence  or  predominance  of  certain  constituents  and  the  comparative  subor- 
dination of  others.  Thus  some  regions,  like  those  of  Christiania  and  Madagascar, 
are  characterized  by  the  abundance  of  alkalis  relative  to  lime,  while  others,  like  that 
of  eastern  Canada,  show  lime  largely  preponderating  over  the  alkalis.  Or,  the 

*  Cf.  De  Stefani,  Boll.  Soc.  Geol.  Hal.,  X,  1891,  p.  550;    also  Washington,  Jour.  Geol.,  V,  1897,  p.  349. 


146 


Analyses  of  Roman  Rocks. 


1 

0 
c/3 

q 

d 
£ 

g 

O 

1 

O 

(3 

O 

i 

O 

ui 

O 

K 

1 
O 

K 

6 
U 

6 
P 

q 

9 

cq 

1 

Inclusive 

(ZrO,     o.«' 

I 

61.88 

18.06 

3.19 

1.38 

0.61 

1.15 

6.89 

6.73 

0.37 

none 

0.69 

0.07 

0.08 

100.52 

]  S03       o  .« 

(Cl         o 

3 

59-79 

19.05 

2.95 

i.  08 

0.36 

1.  19 

6.79 

7.10 

0.24 

none 

0.56 

O.IO 

99.74 

Cl          o'i 

3 

6o.33 

18.27 

2.84 

1.29 

0.38 

1.15 

7-15 

7-30 

0.56 

none 

0.43 

0.04 

.... 

100.17 

a       o.'. 

(ZrO,     o.: 

4 

59-24 

18.63 

3-30 

1.  20 

0.12 

3.06 

4.87 

9.14 

0.86 

none 

0.56 

0.15 

0.06 

100.54 

(Cl3       a. 

5 

61.62 

i8.ii 

2.36 

1.28 

0.56 

1-44 

5-77 

7.60 

0.78 

none 

0.87 

0.13 

100.67 

Cl 

6 

58.08 

19.11 

3-55 

1.  00 

1-05 

2.84 

8.86 

0-54 

O.II 

none 

0.82 

O.2O 

99.92 

7 

57.58 

19-39 

3-22 

1.62 

I.I? 

I'.ls 

3.12 

8.68 

0.50 

o.44 

0.31 

0.21 

100.49 

ci  o 

8 

57-50 

18.80 

4-37 

0.62 

1.  2O 

3-84 

3-16 

8.39 

0.61 

0.38 

0.50 

0.28 

100.21 

5  ci       o., 

<  MnO    o.. 

9* 

56.  10 

20.75 

1.71 

3.19 

I.I4 

3.53 

2.86 

10.47 

0.70 

0.30 

none 

0.65 

o.  24 

10 

55-17 

19.60 

3-27 

2.74 

1-58 

3-73 

3.37 

9.58 

0.99 

none 

0.69 

O.2O 

99.82 

ii 

57-6o 

19-43 

3.49 

1.92 

I.  06 

4-17 

3-55 

8.71 

0.32 

0.32 

0.46 

0.20 

100.50 

MnO     o 

,  ZrO,     o., 

13* 

55-07 

20.83 

a.  13 

1-99 

1.  00 

3-37 

4.00 

8.65 

0.77 

o-59 

none 

0.82 

O.I9 

O.2O 

99-73 

)  SO3      tra  , 
\Ce,C,   o.,' 

SrO      o.c 

13 

55-87 

20.85 

2-34 

I.IO 

0.48 

3-07 

4.81 

10.49 

0.34 

none 

0.79 

O.II 

O.O9 

100.55 

5  ZrO,     o., 

14* 

50.25 

21.41 

1.76 

1.82 

0.31 

4-48 

5-16 

11-33 

0.62 

0-34 

0.33 

0.57 

0.12 

0.13 

99.86 

j  Cl  3       o.: 

(  SrO       tra 

IS* 

59-41 

19.06 

1.87 

3-42 

2.05 

4-09 

2.58 

5-29 

0.64 

0.91 

none 

I  .00 

O.29 

100.  61 

16 

57-32 

19.07 

3.31 

2-35 

1.  60 

3-82 

3.22 

9-iS 

0.57 

none 

0.61 

O.I? 

O.I2 

100.21 

S'63""tr'a' 

17 

c  e    Aft 

14.63 

1-34 

4.50 

7  .  90 

6.69 

i.  79 

6.63 

o.  23 

o.  15 

none 

0.53 

O.36 

18 

OJ  '  V 

7    80 

f,    -X 

o  18 

19 

55.85 

18.34 

3-77 

4-37 
1.88 

1-73 

3-84 

i  -35 
3-39 

u  .  3° 
8.77 

1.14 

none 
none 

0-59 

O.3O 

0.38 

none 
0.17 

100.61 
99.90 

(  ZrO,     noi 
I  SOj       o.c 

(ZrO,      o.i 

30 

55-a* 

18.78 

2.69 

2.86 

1.68 

4-61 

3-13 

8-45 

0.09 

none 

0.71 

O.33 

0.18 

99.6i 

<  SO3      noij 

(  S           not 

(  ZrO,     trai' 

31* 

53.14 

15-91 

2-15 

5-19 

5." 

8.55 

1.93 

7-24 

O.IO 

0.09 

none 

1.22 

0.34 

O.3O 

100.13 

1  SO3       o.c 

(ZrO.      o.i 

33* 

55-22 

18.43 

2.  02 

3-10 

2.75 

5-70 

3-50 

7-58 

0.46 

0.29 

none 

0.87 

0.21 

O.O7 

100.41 

^SO,       o.i 

(  Cr,O.   not 

33 

56.75 

18.03 

2.23 

3-04 

2.  O2 

4.68 

4-85 

5-92 

o.i  8 

none 

1.24 

0.34 

09.38 

Cl          o.-. 

24 

M92 

19.60 

2-45 

3.09 

I  .  OO 

5.00 

3.52 

6.87 

2.08 

0.65 

trace 

99-93 

Cl          o.c 

•  /* 

ZrO,     o.c 

»5* 

52.37 

20.89 

I.  21 

4-44 

1-74 

5.08 

2.OO 

7-47 

1.46 

0.51 

none 

I-IS 

0.51 

0.16 

99.96 

SO3      nor, 

Cl         noi 

a6* 

50.86 

18.48 

4-03 

3-45 

a-55 

7-77 

3.23 

7-iS 

1.51 

o.i  6 

none 

1.  00 

0.46 

0.17 

99.84 

ZrO,     o.: 
SO3      not 

ZrO,     no:> 

37* 

51.21 

18.28 

3-07 

4.19 

3-47 

7.86 

2.49 

6.60 

0.56 

0.16 

none 

1-43 

0.35 

O.IO 

99-77 

SO3       no: 

38 

55.69 

17.87 

4.07 

3.26 

3.41 

6.87 

2.89 

4.41 

0.17 

none 

1.02 

0.19 

99.85 

(  ZrOj     o.o 

29* 

54.83 

19-59 

1.66 

3-04 

i-49 

4-05 

3.93 

10.40 

0.77 

0.49 

none 

0.73 

0.17 

0.15 

100.30 

SOj      non 

<  ZrOa     o.c' 

30* 

51.20 

21.21 

2.38 

3-67 

1.99 

5.42 

2.  II 

10.63 

0.28 

O.IO 

none 

0.74 

0.36 

0.33 

100.45 

i  SO3      trac 

(  ZrOa     trac 

31* 

50.68 

19.46 

3-96 

2.51 

2.34 

6.78 

2.6l 

9.38 

0.46 

0.16 

none 

0.89 

0.33 

0.15 

99.61 

•{  SOj      non 
(  CuO     non; 

32* 

50.36 

17.62 

4.80 

2-53 

3-27 

7.61 

1-99 

9-39 

1.19 

none 

1.09 

0.40 

100.25 

•  •••  •  •  •  *vj 

33 

50.24 

18.43 

2-54 

5-65 

3-65 

7.83 

2.45 

7-45 

0.36 

none 

1.19 

0.47 

0.39 

100.55 

(  ZrO,     non 
i  SO  3      non 

i  ZrO,     o.oj 

34* 

47.65 

18.13 

3.63 

6.48 

4.19 

9.01 

2.78 

7-47 

0.13 

O.II 

none 

I-I3 

0.50 

0.24 

100.47 

i  SO3      trac 

35 

47.89 

17.87 

4-93 

3-64 

3-68 

8.70 

3.60 

8.23 

0.65 

none 

0-77 

0.36 

0.38 

99.68 

I  ZrOa     o.ci 

1  SO3       o.o 

36* 

47.30 

17.66 

3-51 

4-50 

4.20 

9.52 

2.25 

7-63 

0.73 

0.57 

none 

1.19 

0.58 

0.19 

99.76 

ZrO,     o.o* 
SO3      non 

ZrO,     trac 

37* 

48.10 

17.56 

3.48 

6.10 

4.27 

8.16 

3.67 

7-90 

O.I3 

0.04 

none 

1.41 

I.OI 

0.08 

99-90 

•  SO3       trac 
CuO     non 

38* 

47-71 

I7.6l 

3.46 

5-68 

4.80 

9-42 

2-75 

7.64 

trace 

none 

0-37 

0.77 

0.26 

99-53 

ZrO,     o.oj 
SO3       non 

,  ZrO,     o.o 

39* 

47-39 

M-79 

3.10 

5-08 

6.77 

ii.  61 

1.49 

6.93 

0-77 

0.38 

none 

1.41 

o.4S 

0.15 

100.35 

\  SO3      non 
;Ce,O3  o.o 

(  SrO       o.o 

ZrO  2     trao 

40* 

44.89 

12.73 

3-31 

4-35 

13.71 

13.95 

1.02 

3-66 

i-59 

0.37 

none 

o.95 

0.23 

0.08 

99-77 

Cr,O3   o.o 

i  ZrO,     trac 

41* 

46.24 

14.43 

4.06 

4.36 

6.99 

13-^4 

1.65 

6-37 

0.78 

0-57 

none 

1.17 

0.41 

0.13 

100.41 

SO3       o.o 

(  ZrO,     o.c 

42* 

46.27 

17.25 

3-94 

3-23 

5.08 

11.40 

3.67 

8.58 

o.37 

0.13 

none 

i.  ii 

0.39 

O.IO 

100.  61 

1  SO3      trac 
(  SrO       o.o 

43* 

47-05 

17.08 

3-04 

4.89 

5.67 

10.63 

1.61 

7-52 

0.71 

0.22 

none 

...5 

0.17 

0.09 

99-90 

,  ZrO,     o.o 
\  SO3       non 
)  CuO     non 
t  NiO      o.o 

44 

45-99 

16.56 

4-17 

5-38 

5-30 

10.47 

3.18 

8.97 

o.45 

none 

0.37 

0.56 

0.25 

100.65 

SrO       non' 

A  nalyses  of  Roman  Rocks. 


147 


District. 

Locality. 

Old  name. 

New  name. 

Subrang. 

j      mpanian  

2      .mpanian  
.rapanian  

Marecocco,  Ischia  

Augite-trachyte,  Ischia  type.  .  .  . 

Phonolitic  trachyte,  Cuma  type  . 
Phonolitic  trachyte,  Cuma  type. 

Phonolitic  trachyte,  Cuma  type  . 

Trachyte-obsidian,  Rotaro  type. 
Vulsinite,  Bolsena  type  

Tschial  nordmarkose-phlegrose.. 

Cumal  nordmarkose-phlegrose  .  . 
Cumal  nordmarkose-phlegrose  .  . 

Cumal  phlegrose  

I-  S-i-3-4. 

I-  5-  1  -3-4- 
I-5.I-3-4- 

I.5.I.3. 
I.S.i.3. 

I.  5.2.  2. 

I.  5-  2.  3. 
I.  5.  3.  a. 

I.  5.  2.2. 

I.  5.  a.  2. 
1.5.2.3-3. 

1.6-5.2.3. 
1.6.1.3. 

II-I.7.I.3. 

11.4.3.3. 
I-II.  5.  2.  a. 
II.  5.2.2. 

II.  5.  2.  2. 
II.  5.  2.  2. 

II.  5.  2.  2. 
11.6-5.3.2. 

II.5.3.3. 
II.5.2.3. 

U.S.  2.3. 
11.5.2-3.3. 

.5.2-3.3. 

11.5.3.2. 
11.5.3.3. 

H.  6.3.2. 
11.  6.  2.  2. 

II.  6.  2.  2. 

n.  6.  2.2. 
II.  6.  2.  2. 

n.  7-  2.  2. 

II.  7.2.  a. 
II.  7.  3.  2. 

n.  7.2.2. 

II.8-7.2.2. 
III.8-7.3.3. 

in.  7.  3.  2. 

III.  8.  3.  3. 
III.  8.2.2. 

III.  8.  2.  2. 
HI.  8.2.2. 

Monte  di  Cuma,  Phlegrean  Fields 
Monte  Nuovo,  Phlegrean  Fields 

Block  in  tuff,  Vico  Volcano  
Monte  Rotaro,  Ischia  

,     impanian  

Rotaral  phlegrose 

Below  castle,  Bolsena  

Bolsenal  vulsinose  

7     impanian  
g    impanian  

Caprara  ,  Astroni  Volcano,  Phle- 
grean Fields. 

Pagliaroni,     Astroni    Volcano, 
Phlegrean  Fields. 

Sorgente     di     Grignano,    Vico 
Volcano. 
Below  San  Rocco,  Vico  Volcano. 
Rotondella,     Astroni    Volcano, 
Phlegrean  Fields. 

Proceno,  near  Acquapendente.  . 

Poggio  Muratella,  Lake   Brac- 
ciano. 

Osteria    Tavolato,    Via  Appia 
Nuova. 

La  Cava,  Cimino  Volcano  

Vulsinite,  Bolsena  type  

Bolsenal  vulsinose  

Vulsinite,  Bolsena  type  

Bolsenal  vulsinose 

Leucite-trachyte,  Viterbo  type.  .  . 

louche-trachyte,  Viterbo  type.  .  . 
Vulsinite,  Bolsena  type  

Viterbal  vulsinose  

^  .' 

Viterbal  vulsinose  

'    "   'an 

Bolsenal  vulsinose-pulaskose.  .  .  . 
Paglial  procenose-pulaskose  

Sabatinal  beemerose  

Leucite-trachyte,  Paglia  type  .  .  . 
Leucite-phonolite,  Sabatino  type. 

Leucite-tephrite,  Tavolato  type. 
Biotite-latite,  Soriano  type  

Tavolatal  janeirose-appianose.  .  . 
Sorianal  harzose  

15*  ;.    •  • 

Near  Vetralla,  Vico  Volcano  — 
Fontano    Fiescoli,  Cimino  Vol- 
cano. 
La  Colonetta,  Cimino  Volcano.  . 

Bagnorea,  near  Orvieto  

Vulsinite,  Arso  type  

Arsal  vulsinose-ciminose  

Ciminite,  Fiescoli  type  
Ciminite,  Fiescoli  type  

Fiescolal  ciniinose  
Fiescolal  ciminose  

Leucite-trachyte,  Bagnorea  type. 
Leudte-trachyle,  Bagnorea  type. 

Leucite-tephrite,  San    Martino 
type. 

Vulsinite,  Arso  type  

Bagnoreal  ciminose  ... 

Monte  Venere,  Vico  Volcano  .  .  . 
San  Martino,  Vico  Volcano  

Poggio  Cavaliere,  Vico  Volcano. 
L'  Arso,  Ischia  

Bagnoreal  ciminose  

*°i 

Martinal  vicose-ciminose  

Arsal  monzonose  

13  Campanian  
,4  Campanian  

vc  Ciminian.  .  . 

Vulsinite,  Arso  type  
Vulsinite,  Bolsena  type  

Arsal  monzonose  
Bolsenal  monzonose  

Astroni      Volcano,    Phlegrean 
Fields. 
Croce    di  San    Martino,   Vico 
Volcano. 

Below  Orchi  

Leucite-tephrite,  Viterbo  type  .  . 

Leucite-trachyte,  Teanotype... 

Leucite-tephrite,  Orvieto  type.  .  . 
Biotite-latite,  Monfina  type  

Leucite-tephrite,  Viterbo  type..  . 
Leucite-tephrite,  Viterbo  type.  .  . 

Leucite-tephrite,  Bagnorea  type. 

Leucite-tephrite,  Bagnorea  type  . 
Leucite-tephrite,  Orvieto  type.. 
Leucite-tephrite,  Vesuvius  type  , 
Leucitite,  Galera  type  

Foglianal  ciminose-auruncose  .  . 

Teanal  ciminose-auruncose  
Orvietal  aur  uncose  

fl 

Toscanella  

Monte      Santa    Croce.    Rocca 

Monfina. 

Monte  FogUano,  Vico  Volcano.  . 
Monte  San  Antonio  

Monfinal  shoshonose  

Foglianal  vicose  

15 

Foglianal  vicose  

Poggio  Cotognola,  Bracciano.  .  . 

Madonna  del  Riposo,  Bracciano. 
Monte  Cavallo,  near  Qrvieto  .  .  . 

Lava  of  1872,  below  Observa- 
tory, Mount  Vesuvius. 
Crocicchie,  S.  of  Lake  Bracciano 

Arcioni,  Rocca  di  Papa  

Bagnoreal  vicose  

Bagnoreal  vicose  

Orvietal  vicose  
Vesbal  braccianose  

5    Campanian  

Galeral  braccianose  

Hernical  braccianose  

1  ^ 
j    Campanian  

3    Campanian  

Lavaof  1003,  Valle  del  Inferno, 
Mount  Vesuvius. 

Lava  of  1631,  La  Seal  a,  Mount 
Vesuvius. 

Monte  Jugo,  near  Montefiascone 

Fiordine,  near  Montefiascone.  .  . 
Monte  Rado,  E.  of  Lake  Bolsena 

Leucite-tephrite,  Atrio  type  
Leucite-tephrite,  Scala  type  

Leucitite,  Galera  type  
Leucite-basanite,  Fiordine  type. 

Atrial  braccianose  .  .  . 

I 

Scalal  vesuvose-braccianose  .  .  . 

Galeral  albanose-jugose  

Fiordinal  fiasconose  

Romal  albanose  

Saccal  albanose  

Pofi          

Saccal  albanose  

Leucitite.    Capo  di  Bove  type, 
"cecilite." 

Boval  albanose  

148  THE  ROMAN  COMAGMATIC  REGION. 

rocks  of  a  region  may  have  the  common  feature  of  high  magnesia,  or  be  uniformly 
quaric  (siliceous)  and  with  lenads  (leucite  and  nephelite)  quite  absent,  and  so  on. 
These  may  be  called  the  absolute  characters,  in  contradistinction  to  the  serial  ones 
to  be  mentioned  presently.  Superposed  on  these  absolute  characters,  if  one  may 
so  express  oneself,  are  the  characters  which  may  be  called  serial,  and  which  consist 
in  a  definite  variation  in  the  amounts  of  one  or  more  constituents  concomitant  with 
a  variation  in  one  or  more  others,  which  variations  may  be  either  in  the  same  or 
opposite  direction.  Thus  it  is  now  well  established  for  many  regions,  and  as  a 
general  (though  not  invariable)  character  of  igneous  rocks,  that  the  alkalis 
alumina  increase  with  increasing  silica,  while  iron,  magnesia,  and  often  lime  decrease. 
Or  again,  the  relations  between  potash  and  soda,  or  between  magnesia  and  ferrous 
oxide,  may  vary  with  variations  in  the  silica. 

Assuming  the  derivation  of  the  rocks  of  any  given  region  from  a  common 
magma  by  differentiation,  it  would  seem  that  the  absolute  characters  are  dependent 
on  the  composition  of  this,  while  the  serial  characters  are  due  to  the  chemical  changes 
brought  about  by  the  processes  of  differentiation,  the  absolute  characters  persisting 
in  spite  of  these.  For  these  reasons  it  would  be  advisable  to  keep  clear  the 
distinction  between  the  two  as  far  as  possible. 

ABSOLUTE  CHEMICAL  CHARACTERS. 

The  range  of  silica  is  considerable,  from  62  to  45  per  cent,  while  if  the  relative 
masses  of  the  various  rock  types  are  taken  into  consideration,  the  range  in  the 
most  abundant  types  would  be  only  from  56  to  47.  In  general  terms,  therefore, 
the  silica  may  be  said  to  be  medium  to  rather  low  and  with  a  rather  narrow  range. 
Alumina  is  generally  high  and  its  most  marked  characteristic  is  its  very  narrow 
range  in  most  of  the  analyses,  only  from  1 7  to  20  per  cent.  In  only  five  is  it  higher, 
and  then  but  slightly  so,  from  20.8  to  21.4,  while  in  five  others  it  is  lower,  from 
1 6. 6  to  1-2.7,  these  ten  being  mostly  of  rather  rare  types. 

The  oxides  of  iron  and  magnesia  are  almost  uniformly  low,  and  the  range 
comparatively  small,  with  a  few  exceptions,  though  not  as  narrow  as  that  of  alumina. 
Lime,  in  contradistinction  to  the  other  bivalent  oxides,  is  almost  uniformly  quite 
high,  and  with  a  very  considerable  range.  Soda,  on  the  other  hand,  is  rather  low, 
and  while  the  total  range  is  considerable  (from  i  .o  to  7.2),  it  is  only  from  i  .5  to 
3 . 5  in  the  majority  of  the  analyses.  Potash,  like  lime,  is  distinctly  high,  and  with 
a  very  considerable  range,  from  3.7  to  11.3,  the  higher  figures  being  almost  the 
highest  ever  recorded  for  this  constituent  in  igneous  rocks,  although  the  range  of 
the  majority  is  only  from  6.4  to  9.6  per  cent. 

Turning  to  the  minor  constituents,  we  find  sufficient  data  for  many  of  them  to 
permit  of  some  general  statements.  Titanium  dioxide  is  almost  uniformly  high, 
ranging  from  0.3  to  1.4,  and  in  most  of  the  rocks  falling  between  o .  6  and  i .  2  per 
cent.  As  this  constituent  was  determined  in  most  cases  by  the  very  reliable  colori- 
metric  method,  the  figures  for  it  may  be  regarded  as  more  trustworthy  than  is 


PETROLOGY.  149 

usually  true  of  European  rocks.  Zirconia,  on  the  other  hand,  is  always  low,  never 
rising  above  o .  i  per  cent  and  sometimes  being  either  absent  or  present  in  unweigh- 
able  traces.  The  content  of  phosphorus  is  also  low,  the  percentages  of  phosphoric 
pentoxide  running  from  o .  i  to  i .  o,  though  figures  higher  than  o .  5  are  exceptional. 
Sulphur  trioxide  and  chlorine  are  seldom  met  with  in  more  than  traces,  and  are 
frequently  absent,  though  about  i  per  cent  of  SO3  is  shown  in  one  very  rare  type. 
Carbon  dioxide  seems  to  be  uniformly  absent,  a  somewhat  remarkable  fact  in  view 
of  the  generally  high  content  in  lime,  and  even  though  fresh  and  unaltered  rocks 
were  chosen  for  analysis. 

The  oxides  of  the  rare-earth  metals,  when  looked  for,  were  found  in  very 
small  amount  both  in  salic  and  femic  types,  and  it  is  possible  that  traces  of  them 
are  uniformly  present  in  most  of  the  rocks.  Chromium  was  only  observed  once, 
in  a  rock  rich  in  olivine,  and  even  here  in  scarcely  more  than  a  trace,  and  was 
apparently  absent  in  all  the  others.  Nickel  seems  to  be  absent,  except  as  possible 
traces  in  the  more  femic  rocks,  and  the  absence  of  copper  in  all  cases  where  tests 
were  made  for  it  is  noteworthy,  as  it  is  often  supposed  that  this  element  is  quite 
frequent  in  Italian  igneous  rocks,*  on  the  basis  of  analyses  by  Specialef  of  lavas 
from  the  Hernican  District  and  by  ForstnerJ  of  lavas  from  Pantelleria.  The  pres- 
ence of  copper  in  the  last  will  be  investigated  shortly  on  material  recently  collected 
by  the  writer. 

Although  for  reasons  explained  elsewhere  manganese  was  not  estimated  in 
any  of  the  rocks,  the  color  of  the  sodium  carbonate  fusion  showed  that  it  is  always 
present.  But  it  probably  seldom  rises  above  0.25  nor  falls  below  0.05  per  cent, 
and  an  average  of  about  o.io  per  cent  would  be  very  close  to  the  truth.  Of  the 
other  elements  the  occurrence  of  baryta  in,  for  this  constituent,  quite  large  amounts, 
is  very  noteworthy.  It  was  found  in  practically  every  case  where  it  was  looked 
for  in  amounts  varying  from  o .  06  to  0.29  per  cent.  Strontia  was,  unfortunately, 
only  occasionally  estimated  and  was  always  found  to  be  in  scarcely  more  than 
traces  when  present,  and  occasionally  absent. 

Connected  with  these  absolute  characters,  or  rather  as  an  extension  or  more 
detailed  statement  of  some  of  them,  are  certain  relations  between  some  of  the  con- 
stituents which  persist  more  or  less  uniformly  throughout  the  whole  series.  The 
first  which  may  be  mentioned  is  the  general  deficiency  of  the  magmas  in  silica. 
In  only  two  or  three  cases  is  there  an  excess  of  silica  over  the  bases,  that  is,  more 
than  sufficient  to  form  the  most  highly  silicated  minerals.  In  the  great  majority 
of  the  rocks,  even  in  those  which  are  perfelic,  there  is  so  little  of  this  that  the 
less  highly  silicated  minerals,  as  leucite,  nephelite,  olivine,  or  melilite,  are  formed, 
either  in  the  norm  or  in  the  mode,  and  usually  in  both. 

A  second  important  relation  is  the  dominance  of  potash  over  soda  (molecu- 
larly),  which  is  very  marked,  and  which  is  shown  in  the  systematic  classification  by 

*Cf.  J.  H.  L.  Vogt,  Zeits.  prakt.  Geol.,  1898,  p.  317. 

t  S.  Speciale,  Boll.  Com.  Geol.  Ital.,  1870,  p.  302. 

t  H.  Forstner,  Zeils.  Kryst.,  VIII,  1883,  pp.  173,   179,   182. 


150  THE  ROMAN  COMAGMATIC  REGION. 

the  abundance  of  dopotassic  subrangs.  The  exceptions  to  this  are  for  the  most  par 
confined  to  the  rarer  rock  types,  the  only  important  one  being  the  rather  commor 
types  of  phlegrose  (1.5.1.3),  which  will  be  discussed  later. 

Another  of  scarcely  less  importance  is  the  dominance  of  alkalis  over  salic 
lime,  that  is  lime  which  enters  into  the  normative  feldspars.  This  is  clearly  sho\ 
by  the  abundance  of  rocks  falling  in  peralkalic  or  domalkalic  rangs,  the  alkali- 
calcic  rangs  being  few  and  mostly  represented  by  rare  types. 

As  a  general  thing,  also,  the  molecules  of  ferrous  oxide  and  magnesia  togethe 
surpass  those  of  total  lime,  and  they  invariably  do  those  of  femic  lime,  while  mole 
ularly  magnesia  is  usually  higher  than  ferrous  oxide,  though  there  are  some  notable 
exceptions  to  this  last  statement.     Similarly  titanium  is  invariably  higher  thar 
phosphorus    and   baryta  higher  than  strontia,  these  being  the  almost  univei 
relations. 

To  sum  up  the  absolute  chemical  characters  of  the  region  then,  we  may  saj 
that  as  regards  the  main  constituents,  silica  is  moderately  high  to  moderately  lo\ 
and  mostly  deficient;    alumina  uniformly  high  and  with  a  remarkably  narrow 
range ;  ferric  and  ferrous  oxides  and  magnesia  low  and  lime  rather  high ;  magnesiz 
usually  dominating  ferrous  iron,  and  both  usually  dominating  lime  and  alwa) 
femic  lime;   the  total  alkalis  high,  with  soda  usually  low  to  moderately  high,  and 
potash  high  to  very  high;  potash  dominating  soda  and  the  alkalis  together  domi- 
nating salic  lime;    as  regards  the  minor  constituents,  titanium  and  barium  are 
high  and  phosphorus  and  manganese  low,  these  being  always  present;   zirconia, 
strontia,  and  the  oxides  of  cerium,  etc.,  are  usually  present,  but  only  in  traces;  sul- 
phur and  chlorine  in  varying  but  small  amounts;  and  chromium,  nickel,  and  cop- 
per are  absent. 

SERIAL  CHEMICAL  CHARACTERS. 

The  most  obvious  of  these  is  that  as  silica  falls  the  iron  oxides,  magnesia,  lime, 
titanium,  and  phosphorus  rise,  while  the  total  alkalis  fall,  alumina  keeping  fairly 
constant.  Except  as  regards  alumina  this  is  the  usual  relation,  obtaining  not  only 
for  comagmatic  regions  (petrographic  provinces),  but  for  igneous  rocks  as  a  whole. 
It  finds  its  expression  in  the  division  of  classes  in  the  quantitative  system  based 
on  the  relative  amounts  of  salic  and  femic  minerals  in  the  norm. 

Within  this  general  variation  some  more  special  ones  of  interest  may  be 
observed.  As  silica  decreases,  or  as  the  femic  components  increase,  the  amount  of » 
potash  relative  to  soda  increases  on  the  whole.  There  are  some  exceptions  to  this, 
but  the  general  law  for  this  region  seems  to  be  amply  substantiated  by  the  great 
majority  of  the  analyses.  It  is,  however,  of  great  importance  to  note  that,  with 
the  exception  of  the  persalic  rocks  in  which  soda  is  high,  this  relative  increase  in 
potash  is  brought  about  rather  by  increase  in  the  potash  relatively  to  the  silica  and 
the  femic  constituents  than  by  decrease  in  the  soda,  the  amount  of  this  varying  only 
within  rather  narrow  limits.  It  is  also  shown  by  the  general  fact  that  the  potash 
is  higher  relatively  to  the  soda  as  the  amount  of  lenic  minerals  in  the  norm  or  of 


PETROLOGY.  151 

leucite  in  the  mode  rises,  the  persalic  rocks  being  very  largely  or  wholly  feld- 
spathic,  while  the  salfemanes  are  almost  entirely  leucitic  and  non-feldspathic. 
In  the  systematic  quantitative  classification  this  is  well  brought  out  by  the  fact 
that  the  majority  of  the  occurrences  of  sodipotassic  subrangs  fall  in  the  persalane 
class,  as  phlegrose  (I.  5.  i.  3),  which  is  quite  common,  pulaskose  (I.  5.  2.  3),  pro- 
cenose  (I.  6.  2.  3),  beemerose  (I.  6.  i.  3),  and  appianose  (I.  7.  i.  3);  while  in  the 
dosalane  class  the  only  sodipotassic  subrangs  are  harzose  (II.  4.  3),  monzonose 
(II.  5.  2.  3),  and  shoshonose  (II.  5.  3.  3),  which  are  decidedly  sporadic,  and  that, 
finally,  no  sodipotassic  subrangs  of  the  salfemane  class  are  known. 

While  the  alkalis  dominate  salic  lime  as  a  rule,  they  do  so  to  a  much  greater 
extent  as  the  salic  components  increase,  as  is  shown  in  the  classification  by  the 
frequent  occurrence  of  peralkalic  rangs  in  the  rocks  of  the  persalane  class,  alkali- 
calcic  rangs  being  observed  only  in  the  dosalane  class,  though  they  do  not  occur 
in  the  more  lenic  orders  of  dosalane  nor  in  the  salfemanes,  the  relative  increase  in 
alkalis,  especially  potash,  and  the  decrease  in  alumina  here  coming  into  play. 

It  has  been  said  above  that  magnesia  dominates  ferrous  oxide  in  the  majority 
of  cases,  and  while  this  is  true  it  is  especially  so  of  the  more  femic  rocks,  notably 
the  salfemanes,  where  the  ratio  of  MgO  to  FeO  is  apt  to  be  high.  This  decreases 
in  the  dosalanes,  and  in  the  persalanes  is  often  about  unity,  magnesia  thus  increasing 
relatively  to  ferrous  oxide  as  the  femic  components  increase  or  as  silica  decreases. 
A  similar  relation  holds  good  of  the  predominance  of  magnesia  and  ferrous  oxide 
over  femic  lime;  this  ratio  increasing  on  the  whole  with  increase  in  the  femic 
minerals  and  lenads. 

As  regards  the  minor  constituents,  both  titanium  and  phosphorus  increase  on 
the  whole  toward  the  femic  end,  though  there  are  some  exceptions.  Zirconia  seems 
to  be  most  apt  to  be  present  in  the  more  salic  rocks,  especially  when  these  are  sodi- 
potassic, and  the  prevalence  of  this  constituent  in  sodic  magmas  is  well  known. 
Barium  shows  less  regular  relations,  but  it  may  be  said  that  in  general  it  is  some- 
what more  abundant  in  the  more  femic  magmas  and  especially  in  those  which  have 
potash  high  relatively  to  soda.  Concerning  strontium  the  data  are  unfortunately 
rather  few,  and  while  there  seems  to  be  no  very  general  rule  in  regard  to  it,  there 
are  indications  that  it  is  more  apt  to  be  present  in  considerable  amount  in  the  more 
femic  magmas  and  especially  in  those  rich  in  lime.  But  the  figures  reported  for 
it  are  uniformly  so  small  that  they  are  not  conclusive.  Chlorine  and  sulphuric 
anhydride  are  most  common  in  the  sodipotassic  magmas,  especially  when  persalic. 

Normative  Characters. 

In  the  annexed  table  are  given  the  norms  of  the  various  rocks,  calculated  from 
the  analyses  in  the  preceding  table,  and  arranged  in  the  same  order.  A  study  of 
these  norms  will  give  further  insight  into  the  chemical  relations  of  the  rocks 
and  their  magmas,  especially  their  relations  to  the  quantitative  system,  and  will 
serve  as  an  introduction  to  the  discussion  of  their  modes  or  the  minerals  actually 
present  in  them. 


152 


THE  ROMAN  COMAGMATIC  REGION. 

Norms  of  Roman  Rocks. 


1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

Q... 

Or. 

39-5 

42.3 

43.4 

53.9 

45.0 

52.8 

51.7 

49  -5 

62.3 

56.7 

Ab... 

46.6 

39.3 

35.4 

26.8 

41.4 

23.  I 

21.5 

25.2 

5-8 

ii.  S 

i9:I 

An  

3.3 

2.8 

1.4 

12.8 

13.6 

12.5 

14.7 

Lc  

Ne  

5-  I 

8.0 

9.5 

7.0 

3.4 

0.6 

2.O 

0.9 

s  8 

HI  

0.5 

0.8 

0.7 

O.  I 

O.2 

O.  2 

Th... 

0.3 

:::: 

C  

Ac  

3-7 

Di  

6  8 

Hy... 

Wo... 

0.5 

0.5 

0.9 

2.4 

O.S 

:::: 

Ol  

0.6 

0.6 

o  8 

2.4 

Am  

Mt... 

i  .9 

1.6 

0.7 

2.6 

4.6 

^    7 

n  

i  .4 

i  .  i 

0.8 

1.  1 

1.7 

i  .5 

0.6 

0.9 

1.2 

1  .4 

0.9 

Hm  

0.6 

1.8 

1.8 

Ap  

O.2 

0.3 

0.3 

0.4 

o.  7 

O.7 

O.  1 

Rest  

0.3 

o  6 

i  .0 

0.8 

1.6 

100.3 

I  00.0 

100.2 

100.7 

100.  6 

IOO.  I 

100.3 

100.3 

100.8 

99.8 

100.7 

12 

13 

14 

15 

16 

17 

18 

19 

20 

21 

22 

n.  8 

0.3 

' 

r... 

51.7 

39  -5 

37.8 

42.8 

45.0 

Ab... 

6  i 

16  8 

An... 

12    8 

18  4 

8.6 

Lc  

28.8 

Ne  
HI  

10.8 

19-3 

19.3 

5.7 

6-3 

5-9 

7-7 

7-8 

Th... 

C..... 

Ac... 

Di  

2.6 

6  o 

6.1 

7.2 

ii.  4 

Hy  

8.1 

3.5 

18.9 

Wo... 

4  6 

OL.     . 

Am  

Mt  

3.0 

2.6 

2    8 

3.0 

11  

i  8 

o  8 

Hm  

o  8 

Ap  

0.4 

0.7 

0.5 

0.6 

0.5 

Rest  

o  6 

i  6 

o  6 

I.O 

99.6 

100.6 

99.9 

100.  6 

IOO.  2 

IOO.  I 

100.7 

IOO.  I 

99.6 

IOO.  2 

100.5 

23 

24 

25 

26 

27 

28 

29 

30 

31 

32 

33 

Q... 

or.:: 

40.6 

42.8 

38.9 

26.  i 

61.7 

37.5 

39  '5 

35.0 

40.0 

Ab  
An... 

32-5 

21-  S 

17.5 

13.6 

6.8 

9.2 

24.6 
22.5 

0.5 
9-5 

17.0 

iV-6 

11.4 

17.0 

Lc..    .. 

19.8 

12    6 

16.1 

3.5 

Ne  
HI  

4-5 

4-3 

5.9 

6.3 

6.4 

I3-I 

9-7 

n.  9 

9.1 

II.  4 

Th... 

C  

Ac... 

Di  

9.  i 

5.9 

13.2 

13.6 

8.5 

8.0 

5.  7 

12.  I 

17.7 

15.5 

Hy... 

Wo... 

0.4 

Ol  

i  .  7 

3'3 

7.2 

i  .0 

3.6 

2.O 

4-  2 

5.7 

Am  .... 

Mt.... 

3-3 

3.  5 

i  .9 

5.8 

4.4 

5-8 

3-5 

5.8 

4.9 

3.7 

H...  . 

2  7 

1.4 

2.  I 

2.3 

Hm.  . 

1.4 

Ap  

0.8 

1.3 

i  .0 

I.O 

0.4 

O.3 

O.Q 

0.7 

I.O 

I.O 

Rest 

o  8 

o.  7 

0.8 

1.2 

0.7 

99.6 

99-9 

IOO.  I 

100.0 

99-8 

99.8 

100.3 

100.4 

99.6 

100.3 

100.7 

PETROLOGY. 

Norms  of  Roman  Rocks — Continued. 


34 

35 

36 

37 

38 

39 

40 

41 

42 

43 

44 

r        

14.5 

20.9 

17.8 

25.9 

n.  a 

8.1 

0.6 

Ab                

\n        

14.7 

12.8 

15.6 

12.2 

13.  i 

13.3 

19.2 

12.8 

9.5 

17.  0 

8.6 

Lc           

23.5 

21.6 

31.8 

16.4 

26  6 

25.5 

16.6 

29  7 

31.8 

Ne    

12.8 

11.9 

IO.2 

I2.S 

12.8 

6.8 

4.8 

7.7 

12.5 

7.7 

9.9 

HI           

Th                 , 

c        

Ac                   

Di         

21.8 

21  .7 

22-S 

17.9 

19.1 

27.8 

16.1 

Hy           

Wo       

01        

5.6 

O.  I 

6.3 

6  i 

15  6 

8  a 

Mt  

3-9 

7-2 

S-i 

3-7 

3-5 

4-4 

4-9 

5-8 

5-8 

4-4 

6.0 

Hm  

° 

AP  
Rest          .        ... 

I-J 

0.5 

0.9 

1-3 

2-3 

1.8 

0.6 

0.3 

1-3 

100.8 

99-6 

99.8 

100.  I 

09.6 

100.2 

99.8 

100.4 

100.7 

IOO.I 

loo.S 

The  first  fact,  and  one  of  the  most  striking  to  be  noted,  is  the  rarity  of  quartz. 
This  mineral  is  found  only  three  times,  in  one  case  in  notable  amount  in  harzose, 
once  in  shoshonose,  and  again  in  negligible  quantity  in  a  ciminose.  The  feldspars 
make  up  very  large  percentages  of  nearly  all  the  norms,  except  in  the  salfemanes 
(39-44)  where  they  are  largely  replaced  by  lenads.  Of  the  feldspars  orthoclase 
and  anorthite  are  the  most  constant,  the  former  vanishing  only  in  albanose 
(III.  8.  2.  2),  and  the  amount  of  anorthite  being  negligible  only  in  phlegrose  (I.  5. 
i.  3),  beemerose  (I.  6.  i.  3),  and  appianose  (I.  7.  i.  3).  It  is  worthy  of  note  that 
the  amount  of  this  anorthite  molecule  is  remarkably  constant,  for  the  most  part 
about  12  or  13  per  cent,  though  it  may  be  higher  or  lower.  Albite,  on  the  other 
hand,  while  present  in  large  amount  in  the  persalanes  (except  appianose)  and  in 
the  dosalanes  belonging  to  the  perfelic  order  germanare,  disappears  in  the  rocks 
of  this  latter  class  as  soon  as  the  lenads  increase  to  notable  amounts  in  vicose  (II. 
6.  2.  2),  and  from  this  on  it  is  n^t  found  in  the  norm.  With  few  exceptions  it  is 
very  subordinate  to  orthoclase,  though  this  is  compensated  for  in  many  cases  by 
the  amount  of  nephelite. 

Correlated  with  the  poverty  of  the  magmas  in  normative  quartz  (excess  of 
silica)  is  the  almost  invariable  presence  of  lenads,  either  nephelite  or  leucite  or 
both.  Apart  from  the  norms  which  are  quaric  (15,  18,  and  28),  and  where  they 
can  not  exist,  lenads  are  absent  only  in  the  norm  of  one  rock,  No.  17.  Leucite  is 
absent  from  the  norms  of  all  the  persalanes  except  appianose,  as  well  as  from  the 
perfelic  dosalanes,  and  begins  to  appear  in  vicose  (in  which  concomitantly  norma- 
tive albite  is  absent),  whence  to  the  most  femic  rock  it  is  a  constant  normative 
constituent.  Nephelite,  on  the  other  hand,  is  present  in  all  the  norms,  except  the 
five  spoken  of  above,  in  which  lenads  are  not  present,  and  from  vicose  to  albanose 


154  THE  ROMAN  COMAGMATIC  REGION. 

it  is  the  only  sodic  mineral.  Of  the  other  lenic  minerals,  halite  and  thenardite 
(existing  modally  in  sodalite  and  noselite  or  haiiyne)  are  found  only  in  the  persa- 
lanes,  and  of  these  only  in  the  sodipotassic  subrangs,  phlegrose,  beemerose,  and 
appianose.  In  the  dosalanes  and  salfemanes  they  seem  to  be  entirely  lacking. 

The  femic  constituents  need  less  discussion,  as  they  are  present  in  much  smaller 
amount  than  the  salic.  This  is  especially  true  of  the  persalanes,  in  which  the 
amounts  of  femic  minerals  are  negligible.  Of  these  it  may  only  be  said  that  acmite 
occurs  but  once,  and  that  rocks  falling  in  this  class  are  about  the  only  ones  which 
show  normative  wollastonite,  this  being  present  generally  when  the  subrang  is 
sodipotassic. 

Diopside  is  constantly  present,  being  absent  only  in  two  cases,  while  norma- 
tive hypersthene  is  rare,  but  this  last  can  not,  of  course,  be  present  in  the  norm 
along  with  the  lenads.  Olivine  is  almost  as  constantly  present  as  diopside,  and 
is  absent  only  when  there  is  normative  quartz  or  wollastonite.  The  amounts  of 
both  of  these  vary  considerably,  but  increase  gradually  though  irregularly  as  the 
magmas  become  more  femic.  Magnetite,  ilmenite,  and  apatite  are  always  present, 
but  in  small  amounts,  and  each  varies  within  a  very  narrow  range. 

Relations  of  Norm  and  Mode. 

The  readjustments  of  the  molecules  of  the  norm  to  form  the  modes  of  the  several 
types  have  been  spoken  of  in  each  case  in  the  preceding  pages,  but  it  will  be 
instructive  to  summarize  the  main  features. 

In  leucite-free  types  the  readjustments  are  almost  always  of  minor  impor- 
tance and  the  modes  are  essentially  normative.  In  these  the  augite  is  the  chief 
disturber  of  the  normative  relations,  this  alferric  mineral  taking  some  of  the  norma- 
tive anorthite,  and  still  less  of  the  magnetite,  ilmenite,  and  nephelite,  to  combine 
with  the  normative  diopside.  But  as  these  non-leucitic  types  are  only  found  in 
the  persalanes  and  dosalanes,  where  the  femic  minerals  are  either  negligible  or 
subordinate,  and  as  the  amount  of  anorthite  involved  is  only  about  one-fifth  of  the 
diopside  (and  that  of  the  other  minerals  involved  much  less),  the  disturbance  is 
seldom  serious.  It  is  noteworthy  that  wollastonite  is  more  apt  to  be  present  in 
the  norms  of  the  persalane  rocks  than  hypersthene  or  olivine,  and  this  is  probably 
connected  with  the  presence  of  this  mineral  in  the  norm  of  the  augite  of  the  region 
when  it  is  calculated  into  standard  minerals.  The  high  wollastonite  in  the  norm 
of  the  tavolatal  appianose  [hauynitic  leucite-tephrite,  ta volatile]  is  of  interest  in 
connection  with  the  occurrence  of  garnet  in  this  rock. 

In  a  few  types  characterized  by  the  presence  of  biotite  readjustments  are  needed 
for  this  mineral,  involving  normative  leucite,  olivine,  and  also  anorthite  to  furnish 
the  extra  alumina  in  exact  calculations.  But  here  also  the  divergences  from  the 
norm  are  notable  only  in  one  or  two  cases.  The  reciprocal  exclusion  of  biotite 
and  leucite,  so  commonly  observed  elsewhere,  is  also  evident  throughout  the  region. 
Though  there  are  a  few  cases  where  the  two  minerals  do  occur  together,  as  in  the 


PETROLOGY.  155 

type  of  teanal  auruncose,  it  is  generally  observed  that  in  rocks  derived  from  identical 
or  very  similar  magmas,  while  some  contain  biotite  the  others  do  not  contain  this 
mineral,  but  leucite  and  olivine  replacing  it.  That  is,  the  alferric  biotite  molecule 
has  been  split  up  actually  into  the  simpler  salic  and  femic  ones. 

By  far  the  most  important  set  of  readjustments  is  that  involved  in  the  formation 
of  leucite  from  magmas  whose  norms  do  not  show  this  mineral.  This  topic  will 
be  discussed  at  some  length  on  a  later  page,  but  it  may  be  pointed  out  here  that 
in  these  rocks  the  presence  of  leucite  in  such  magmas  involves  the  taking  away  of 
silica  from  normative  orthoclase,  which  goes  to  form  albite  molecules  from  the 
normative  nephelite,  which  albite  enters  modally  into  soda-lime  feldspar.  In  a 
few  cases  some  of  the  normative  olivine  seems  to  be  involved,  this  becoming  hyper- 
sthene  which  enters  into  modal  augite.  But  this  is  exceptional,  and  in  the  majority 
of  instances  the  readjustments  take  place  between  normative  orthoclase  and  nephel- 
ite, forming  leucite  and  albite  in  the  mode,  the  normative  minerals  disappearing 
either  in  whole  or  in  part. 

The  generally  close  correspondence  between  norm  and  mode  among  the  non- 
leucitic  rocks  of  the  region,  and  the  simple  molecular  readjustments  involved  in 
the  formation  of  leucite,  are  of  interest  as  bearing  on  the  propriety  of  the  choice 
of  the  so-called  "standard  minerals"  in  the  quantitative  classification.  It  has 
already  been  pointed  out*  that  in  the  majority  of  igneous  rocks  there  is,  on  the 
whole,  a  close  correspondence  between  the  norm  and  the  mode  in  the  majority  of 
rocks;  that  is,  that  the  readjustments  of  the  norm  needed  for  the  mode  are  mostly 
of  small  amount  and  the  modes  normative.  In  his  recent  monumental  work  on 
Mont  Pelee  Prof.  A.  Lacroixf  points  out  that  the  standard  minerals  are  "among 
those  which  are  formed  by  (simple)  igneous  fusion  followed  by  slow  cooling,  at 
the  expense  of  those,  such  as  the  amphiboles,  micas,  and  garnets,  which  are  not 
so  formed." 

The  practical  absence  of  amphiboles,  biotites,  and  the  more  complicated 
pyroxenes  is  certainly  a  striking  feature  of  the  region,  and  it  would  seem  that  for 
this  igneous  complex,  at  least,  the  choice  of  standard  minerals  is  appropriate  and 
expresses  the  modal  facts  as  closely  as  any  such  selections  could  do. 

Mineralogical  Characters. 

Introduction. — That  the  mineralogical  composition  or  mode  of  igneous  rocks 
is  determined,  within  limits,  by  the  chemical  composition,  subject  to  the  physical 
conditions  obtaining  during  solidification,  is  so  well  known  that  the  statement  is 
almost  a  truism.  But  it  is  found,  furthermore,  that  in  a  comagmatic  region  the 
minerals  present,  or  at  least  some  of  them,  show  certain  more  or  less  common 
peculiarities,  which  may  be  regarded  as  characteristic  of  the  region,  and  which 
are  usually  looked  upon  as  evidences  of  "consanguinity"  only  second  to  the  chemical 

*  Cross,  Iddings,  Pirsson,  and  Washington,  op.  cit.,  p.  151;  and  Washington,  Prof.  Paper  U.  S.  Geol.  Surv. 
No.  14,  1903,  p.  69. 

t  A.  Lacroix,  La  Montagne  Pette  et  ses  Eruptions,  Paris,  1004,  p.  529 


156  THE  ROMAN  COMAGMATIC  REGION. 

characters.  In  addition  to  the  presence  of  certain  minerals  and  their  peculiarities, 
the  absence  of  others  which  might  be  expected  to  be  present,  so  far  as  the  magmatic 
or  normative  characters  permit,  is  another  mineralogical  characteristic  of  comag- 
matic  regions. 

Soda-orthoclase. — A  constant  feature  of  the  alkali-feldspars  of  the  region  is 
that  pure  orthoclase  seems  never  to  occur,  notwithstanding  the  highly  potassic 
character  of  the  magmas,  the  orthoclase  molecule  being  invariably  accompanied 
by  notable  amounts  of  albite,  so  that  the  alkali-feldspar  is  a  soda-orthoclase.  The 
orthoclase  molecule,  however,  is  nearly  always  present  in  larger  amount  than  that 
of  albite,  the  composition  varying  from  OreAbj  to  (rarely)  OrnAbj.  As  a  common 
character  it  is  notable  that  the  microcline  structure  is  never  seen,  and  microper- 
thitic  intergrowths  of  orthoclase  and  albite  are  exceptional,  while  zonal  structure 
is  uncommon,  the  sections  of  the  soda-orthoclase  usually  appearing  uniform  and 
homogeneous. 

In  habit  the  soda-orthoclase  occurs  either  as  phenocrysts,  often  of  large  size* 
and  either  prismatic,  parallel  to  the  clinoaxis  a,  or  tabular,  parallel  to  the  clino- 
pinacoid  &(oio),  or  as  small  prismatic  laths  (parallel  to  the  axis  a)  in  the  ground- 
mass.  It  may  also  rarely  form  an  interstitial  cement,  and  in  some  types  occurs 
thus  as  a  mantle  of  later  growth  around  phenocrysts  of  soda-orthoclase  or  soda- 
lime  feldspar  and  oriented  parallel  with  them. 

It  occurs  most  abundantly  in  the  persalanes,  especially  in  the  phlegrose  rocks, 
is  common  in  the  dosalanes,  except  in  braccianose,  where  it  is  wanting,  and  is  not 
found  in  the  salfemanes.  Its  presence  in  the  mode,  therefore,  would  seem  to  be 
directly  connected  with  the  variations  in  the  silica  and  the  femic  components. 

Soda-lime  feldspar. — The  composition  of  this  varies  considerably,  from  ande- 
sine,  Ab3An2,  to  an  almost  or  wholly  pure  anorthite,  but  in  the  majority  of  the 
types  it  approximates  to  a  labradorite,  AbtAn2.  Multiple  twinning  is  common, 
except  in  the  groundmass  laths  of  some  types,  according  to  the  Carlsbad  and  albite, 
and  less  often  the  pericline  laws.  Zonal  structure  is  not  frequent,  but,  as  said 
above,  in  some  types  phenocrysts  of  labradorite  or  anorthite  are  surrounded  by 
mantles  of  orthoclase  oriented  like  the  nuclear  crystal,  that  is,  with  the  vertical 
axes  and  the  clinopinacoids  b  (oio)  parallel. 

The  habits  of  these  feldspars  resemble  those  of  the  soda-orthoclase,  but  it 
may  be  noted  that  the  phenocrysts  of  plagioclase  are  usually  smaller  than  those  of 
orthoclase,  they  are  apt  to  be  clustered  into  groups,  and  that  groundmass  laths  are 
less  common.  These  feldspars  are  rarely  known  to  occur  as  an  interstitial  cement. 

The  modal  occurrence  of  the  soda-lime  feldspars  is  exactly  the  opposite  of 
that  observed  in  the  case  of  the  soda-orthoclase.  They  are  wanting  in  rocks  belong- 
ing to  phlegrose  and  other  peralkalic  subrangs  of  persalane,  rare  generally  in  the 
rocks  of  the  persalane  class,  most  common  in  the  dosalanes,  especially  in  braccian- 
ose, where  they  constitute  the  only  feldspars,  but,  like  the  soda-orthoclase,  are  usually 
absent  from  the  salfemic  rocks. 


PETROLOGY.  157 

Leucite. — The  abundance  of  this  mineral  is  one  of  the  most  characteristic 
modal  features  of  the  region,  and  has  made  the  volcanoes  of  the  Bolsena- Vesuvius 
line  famous  in  petrography.  Apart  from  this  it  can  not  be  said  that  the  mineral 
offers  any  features  specially  peculiar  to  the  region,  as  its  characters  here  are  those 
of  many  of  its  occurrences.  In  certain  magmas,  especially  those  belonging  to 
vulsinose,  ciminose,  and  vicose,  it  forms  well-formed  phenocrysts  of  very  large 
size,  up  to  5  or  i o  cm.  in  diameter,  which  are  characteristic  of  the  viterboid  habit. 
It  may  also  occur  in  the  groundmass  as  small  crystals,  sometimes  well  shaped 
and  again  mere  spheroids,  and  rarely  as  anhedral  patches.  The  larger  individuals 
usually  show  the  birefringence  and  twinned  structure,  which  are  commonly  want- 
ing in  the  smaller  groundmass  leucites.  The  larger  ones  also  are  usually  relatively 
free  from  inclusions,  while  the  characteristic  regular  arrangement  of  small  inclusions 
in  the  groundmass,  leucite  microphenocrysts,  and  the  frequent  skeletal  forms  of 
these,  are  well  known. 

The  occurrence  of  leucite  in  relation  to  the  composition  of  the  magma  is  an 
interesting  topic,  the  discussion  of  which  will  be  taken  up  on  a  later  page. 

Nephelite. — It  is  a  somewhat  striking  fact  that,  while  nephelite  is  found  in  the 
norms  of  nearly  all  the  rocks,  sometimes  to  a  very  large  amount,  it  is  seldom  present 
modally  in  more  than  small  quantities,  not  over  10  per  cent,  most  usually  from  i 
to  5,  and  is  often  absent  entirely,  the  soda  entering  the  albite  molecule  to  form  a 
soda-lime  feldspar.  The  typical  occurrence  of  nephelite  in  the  Roman  Region  is 
in  the  form  of  an  interstitial  cement,  the  last  product  of  crystallization.  Euhedral 
or  even  subhedral  crystals  are  excessively  rare,  except  in  crevices  in  the  rocks, 
where  small,  short  prisms  are  fairly  common  in  some  types. 

Sodalite  minerals. — While  these  are  met  with  in  the  rocks  of  the  region,  their 
occurrence  can  not  be  regarded  as  characteristic  of  it.  For  the  most  part  their 
amount  is  very  small,  scarcely  more  than  accessory,  except  in  tavolatal  appianose, 
where  haiiyne  is  one  of  the  main  constituents.  Both  sodalite  and  hatiyne  are  found, 
but  the  former  seems  to  be  the  more  common. 

Their  occurrence  is  practically  confined  to  the  persalane  rocks,  and  especially 
to  the  sodipotassic  subrangs,  the  sodalites  observed  by  De  Lorenzo  and  Riva  in 
the  bolsenal  vulsinoses  [vulsinites]  of  the  Astroni  Volcano  being  very  rare  and 
sporadic,  and  corresponding  to  the  occurrence  of  leucite  in  the  blocks  of  cumal 
phlegrose  [phonolitic  trachyte]  at  the  Vico  Volcano  or  in  the  arsal  monzonose  [cim- 
inite]  at  L'Arso. 

Augite. — This  alferric  mineral  is  the  one  which  is  most  characteristic  of  the 
region,  and  from  the  point  of  view  of  the  alferric  and  femic  minerals  the  region  is 
decidedly  an  augitic  one,  as  it  is  eminently  a  leucitic  one  from  the  salic  point.  It 
is  invariably  present  in  greater  or  less  amount  in  all  the  rocks,  without  exception, 
the  amount,  of  course,  varying  with  the  character  of  the  magma  as  to  the  relation 
of  salic  and  femic  components. 


158  THE  ROMAN  COMAGMATIC  REGION. 

The  composition  would  seem  to  be  very  uniform  and  may  be  represented  by 
the  analysis  on  p.  134,  and  it  is  rather  a  diopside  augite  than  one  with  the  heden- 
bergite  molecule  abundant.  In  some,  but  not  all,  of  the. rocks  belonging  to  sodi- 
potassic  subrangs  the  augite  carries  some  aegirite,  but  the  amount  of  this  is  usually 
small,  and  aegirite-augite  must  be  regarded  as  rare  or  not  prominent  in  the  region. 

The  color  of  the  augite  is  one  of  its  most  constant  and  characteristic  features- 
While  black  or  very  dark  green  in  the  hand  specimen,  in  thin  sections  it  is  almost 
invariably  either  colorless  or  a  very  pale  grayish,  occasionally  slightly  tinged  with 
green  or  yellow.  Only  in  some  of  the  sodipotassic  rocks  does  the  color  assume  a 
true  green,  when  it  is  pleochroic,  and  evidently  contains  some  aegirite.  But  in  the 
very  great  majority  of  the  rocks,  whether  persalanes,  dosalanes,  or  salfemanes,  the 
typical  color  is  the  very  pale  gray,  which  may  be  said  to  be  characteristic. 

In  this  connection  it  is  worthy  of  remark  that,  though  the  magmas  of  the  region 
are  rather  high  in  titanium,  and  the  augite  analyzed  by  me  contains  2 .85  per  cent 
of  titanium  dioxide,  this  mineral  never  shows  the  peculiar  purple  tones  which  are 
commonly  supposed  to  be  due  to  this  oxide.  A  somewhat  similar  state  of  affairs 
has  been  observed  by  Pirsson  in  the  Central  Montana  Region,  as  will  be  brought 
out  later. 

Hypersthene. — This  mineral  is  one  of  the  rarities  of  the  region,  and  its  absence, 
rather  than  its  occurrence,  is  to  be  regarded  as  characteristic.  It  was  only  observed 
in  the  quaric  and  vitreous  sorianal  harzose,  in  colorless  crystals,  but  is  common  in 
the  quaric  types  of  toscanose  (I.  4.  2.  3)  and  amiatose  (I.  4.  3.  3)  at  Monte  Amiata, 
to  the  north  of  the  Vulsinian  District,  the  magmas  of  which  may  be  connected  with 
those  of  the  Roman  Region. 

Biotite. — This  mineral  is  likewise  rare  in  the  region,  though  more  common 
than  hypersthene.  In  some  types,  as  sorianal  harzose  (II.  4.  3.  3)  [biotite-latite], 
teanal  auruncose  (II.  5.  3.  2)  [biotitic  leucite-trachyte],  and  monfinal  shoshonose 
(II.  5.  3.  3)  [biotite-vulsinite],  all  of  which,  it  is  to  be  noted,  are  alkalicalcic,  it  is 
quite  abundant  and  an  essential  component.  In  other  types,  as  arsal  vulsinose, 
ciminose  (II.  5.  2.  2),  and  monzonose  (II.  5.  2.  3)  [vulsinite],  it  occurs  in  accessory 
amounts,  and  mostly  as  small  interstitial  patches.  It  is  also  met  with,  though  as 
a  rare  accessory,  in  some  of  the  types  of  braccianose  (II.  7.  2.  2)  at  the  Vesuvius 
Volcano.  The  color  is  uniformly  a  pale  brown,  with  marked  pleochroism. 

There  would  seem  to  be  some  connection  between  the  occurrence  of  biotite 
and  that  of  olivine,  the  potential  biotite  molecule  splitting  up  into  leucite  and  olivine. 
A  number  of  instances  of  this  are  mentioned  in  the  descriptions  of  the  types,  and 
the  subject  has  been  discussed  by  the  writer  and  other  petrographers.  On  the 
whole  the  rarity  of  biotite,  rather  than  its  occurrence,  may  be  regarded  as  the 
feature  in  regard  to  it  which  is  most  characteristic  of  the  region. 

Olivine. — Notwithstanding  the  rather  low  silica  of  many  of  the  rocks  this 
mineral  is  decidedly  uncommon,  though  it  is  met  with  in  small  amounts  in  many  of 
the  dosalanes  and  salfemanes,  and  is  characteristic  of  fiescolal  ciminose  (II.  5. 


PETROLOGY.  159 

2.  2)  [ciminite]  and  of  fiordinal  fiasconose  (III.  7.  3.  2)  [leucite-basanite],  in  which 
latter  type  it  is  abundant  as  large  phenocrysts.  It  is  also  a  frequent  accessory  in 
the  various  types  at  the  Vesuvius  Volcano,  but  is  not  as  important  or  constant  a 
constituent  here  as  is  often  supposed.  Its  features  are  the  usual  ones  and  call 
for  no  comment,  as  they  are  not  characteristic  of  the  region. 

Melilite. — While  this  mineral  is,  as  usual,  very  rare,  and  is  present  in  notable 
amount  only  in  the  type  boval  albanose  (III.  8.  2.  2)  [cecilite],  its  occurrence  in 
these  rocks  may  be  noted  as  rather  peculiar  to  the  region.  It  would  seem  to  be 
more  widespread  than  has  been  thought,  but  any  description  of  it  is  needless.  In 
this  connection  its  occurrence  as  an  abundant  and  essential  component  of  the  cellal 
venanzose  (IV.  Is.  i3.  2)  [venanzite]  at  the  small  cone  of  Pian  di  Celle,  San 
Venanzo,  Umbria,  is  worthy  of  attention,  since  this  small  cone  is  possibly  connected 
with  the  main  line  of  volcanoes. 

Absence  of  minerals. — The  most  notable  absentee  is  hornblende,  which  may 
virtually  be  said  not  to  exist  among  the  rocks  of  the  region.  Extremely  small 
amounts  of  a  barkevikitic  hornblende  are  present  in  one  or  two  types,  and  breis- 
lakite  (which  is  usually  referred  to  the  hornblendes)  occurs  in  the  crevices  of  types 
of  braccianose,  vesuvose,  and  albanose  in  the  Sabatinian,  Latian,  and  Campanian 
Districts.  But  these  occurrences  are  exceptional  and  do  not  invalidate  the  general 
statement  that  hornblende,  as  a  true  rock  component,  is  absent  from  the  region. 

In  view  of  the  general  character  of  the  magmas  and  the  fact  that  silica  is 
scarcely  ever  in  excess,  the  absence  of  quartz  is  not  surprising,  though  it  might  be 
looked  for  among  the  harzose  rocks.  The  majority  of  these  are  vitreous  and  show 
no  quartz,  but  it  has  been  observed  among  these  rocks  at  the  Cimino  Volcano  by 
Mercalli,  though  it  does  not  seem  to  be  common. 

Among  the  more  uncommon  minerals,  titanite  is  extremely  rare,  having  been 
observed  in  only  a  few  cases  in  very  small  amounts.  Zircon  is  still  rarer,  and, 
indeed,  no  authentic  occurrences  are  met  with.  The  pyroxenic  and  hornblendic 
minerals  which  contain  the  oxides  of  the  rare  earths,  such  as  lavenite,  rosenbuschite, 
hiortdahlite,  etc.,  are  practically  absent,  lavenite  occurring  as  a  rare  accessory  in 
ischial  phlegrose.  This  subrang  is  sodipotassic,  and  the  general  association  of 
these  minerals  with  magmas  rich  in  soda  is  well  known. 

Textural  Characters. 

Although  the  textural  characters  are  very  largely  determined  by  the  conditions 
of  solidification  and  are  more  remotely,  if  at  all,  dependent  on  the  chemical  com- 
position of  the  magmas,  so  that  they  can  only  rarely  and  in  a  subordinate  degree 
be  considered  as  characteristic  of  a  comagmatic  region,  there  are  certain  features 
which  merit  a  brief  discussion. 

As  regards  crystallinity  the  rocks  of  the  Roman  Region,  which  are  wholly 
effusive  flows,  and  their  tuffs,  are  somewhat  remarkable  for  the  rarity  of  hyaline 
types.  In  the  great  majority  of  cases  the  rocks  are  holocrystalline,  and  this  is 


160  THE  ROMAN  COMAGMATIC  REGION. 

especially  true  of  the  salfemanes  and  the  more  lenic  dosalanes,  among  which  the 
only  hyaline  types  observed  are  small  flows  and  the  borders  of  larger  ones  at  Mount 
Vesuvius.  It  is  possible  or  probable  that  similarly  vitreous  portions  existed  origi- 
nally at  the  other  centers,  but  have  been  removed  by  erosion.  In  any  case  the 
volume  of  these  is  always  small. 

Glass  appears  in  small  amounts  as  the  rocks  increase  in  salic,  and  especially 
feldspathic,  components,  a  little  glass  cement  being  found  in  some  types  of  ciminose, 
vulsinose,  phlegrose,  etc.  The  most  hyah'ne  rocks  belong  to  the  subrangs  phleg- 
rose  (I.  5.  i.  3)  and  harzose  (II.  4.  3.  3),  and  some  types  of  the  former  areper- 
hyaline  obsidians.  It  is  to  be  noted  in  these  cases  that  the  vitreous  types  of  these 
two  subrangs  may  and  often  do  form  massive  flows  and  are  not  confined  to  small 
trickles  of  lava  or  outer  borders  of  large  flows.  Probably  connected  with  this 
tendency  of  these  magmas  to  solidification  before  complete  crystallization  is  the 
tendency  of  the  harzose  to  form  flow  breccias  and  that  of  the  phlegrose  lavas  of 
the  Phlegrean  Fields  to  form  dustlike  tuffs  rather  than  solid  flows.* 

This  tendency  of  the  Roman  magmas  to  form  holocrystalline  rather  than 
hyaline  types  is  probably  connected  with  their  generally  high  content  in  potash 
and  lime.  This  view  is  borne  out  by  the  observed  facts  stated  above,  since  the 
types  of  lenic  orders  of  dosalane  and  of  salfemane,  which  are  uniformly  holocrys- 
talline, are  relatively  the  richest  in  these  two  oxides,  while  the  types  of  harzose 
and  of  phlegrose  are  relatively  the  poorest  and  are  both  sodipotassic.  But  further 
discussion  of  this  topic  is  uncalled  for,  as  it  is  rather  a  general  one  than  character- 
istic of  the  region. 

The  abundance  of  phenocrysts  is  very  noticeable,  though  this  is  not  surprising 
in  view  of  the  fact  that  the  rocks  are  effusive  flows.  It  is  more  interesting  to  observe 
that  the  salic  minerals,  especially  soda-orthoclase  and  leucite,  are  much  more  often 
phenocrystic  than  the  alferric  minerals,  phenocrysts  of  augite  and  of  olivine  sur- 
passing those  of  the  salic  minerals  only  exceptionally.  The  large  size  of  the  pheno- 
crysts of  soda-orthoclase  and  leucite  has  already  been  commented  on.  This 
tendency  of  the  salic  minerals  to  development  in  large  crystals,  while  the  alferric 
ones  are  small,  is  also  evident  in  the  rarity  of  megaporphyritic  types  among  the 
rocks  belonging  to  the  more  femic  divisions,  such  as  braccianose  and  albanose. 
In  these  the  grain  is  uniformly  small  and  megascopic  phenocrysts  are  very  rare 
and  never  attain  large  dimensions. 

Space  Relations. 

For  the  discussion  of  the  space  relations  or  the  geographical  distribution  of 
the  different  magmas  and  rock  types  it  will  be  best  to  collate  and  summarize  the 
statements  made  in  the  preceding  petrographical  descriptions  as  regards  the  occur- 
rences in  the  several  districts.  These  will  be  supplemented,  so  far  as  possible, 
by  the  descriptions  of  other  petrographers,  though  in  most  cases  it  is  difficult,  if  not 

*  Cf.  Deecke,  Gtol.  Fuhrer  durch  Campanien,  Berlin,  1901,  p.  44. 


PETROLOGY.  161 

impossible,  to  translate  these  into  the  terms  of  the  quantitative  classification,  on 
account  of  the  lack  of  reliable  analyses  and  the  statement  of  the  mode  only  in 
qualitative  terms. 

The  data  available  on  which  to  base  the  distribution  of  types  and  the  char- 
acterization of  each  district  vary  widely.  In  some  cases,  as  the  Ciminian,  the 
Latian,  and  the  Campanian  districts,  they  are  satisfactorily  complete  and  accu- 
rate. In  others,  as  the  Vulsinian  and  the  Auruncan,  they  are  as  yet  far  from  being 
complete,  and  in  such  cases  the  descriptions  must  be  regarded  as  provisional. 

VULSINIAN  DISTRICT. 

This  district  is  not  only  the  largest  of  all,  but  is  the  most  complex  in  its  volcanic 
structure,  as  has  been  seen,  and,  furthermore,  shows  the  greatest  diversity  in  its 
rocks.  Of  the  seventeen  subrangs  known  to  be  represented  in  the  Roman  Region 
at  least  eleven  are  found  in  this  district,  and  some  are  confined  exclusively  to  it. 

Rocks  of  the  persalane  class  are  rather  common  and  are  represented  by  types 
of  vulsinose  (I.  5.  2.  2),  pulaskose  (I.  5.  2.  3),  procenose  (I.  6.  2.  3),  and  possibly 
beemerose  (I.  6.  i.  3).  The  types  of  vulsinose  are  non-leucitic,  while  those  of 
the  other  subrangs  mentioned  are  all  leucitic,  this  mineral  forming  usually  large 
phenocrysts.  These  persalic  rocks  are  most  common  in  the  northern  and  north- 
western parts,  as  around  the  Latera  Volcano  and  near  Bolsena.  It  is  perhaps 
worthy  of  note  that  the  subrang  phlegrose  (I.  5.  i.  3),  which  is  elsewhere  common, 
is  apparently  unrepresented  in  the  Vulsinian  District. 

The  subrangs  of  dosalane  are  much  more  abundantly  represented,  and  offer  a 
greater  variety  of  types.  Of  these  subrangs  ciminose  (II.  5.  2.  2)  is  probably  quite 
common  in  the  northern  portions.  The  leucitic  bagnoreal  type  is  found  to  the 
northeast  of  the  lake,  while  non-leucitic  types,  as  the  fiescolal  and  arsal,  seem  to 
occur  to  the  north.  The  alkalicalcic  auruncose  (II.  5.  3.  2)  is  rare  and  is  confined 
to  the  southern  portion,  so  far  as  known.  The  lenic  subrangs,  vicose  (II.  6.  2.  2), 
braccianose  (II.  7.  2.  2),  and  vesuvose  (II.  8.  2.  2),  are  more  abundant,  and  to 
them  belong  many  of  the  flows  to  the  east,  south,  and  southwest  of  Lake  Bolsena. 
The  types  of  these  are  invariably  leucitic,  rarely  with  large  leucite  phenocrysts,  as 
in  the  viterbal  vicose  near  Orvieto,  but  much  more  commonly  quite  without  pheno- 
crysts, or  with  only  microscopic  ones,  as  in  the  orvietal,  galeral,  and  hernical  types. 

Rocks  belonging  to  salfemane  are  almost,  if  not  quite,  as  abundant  as  those 
in  dosalane,  but  they  are  more  monotonous  in  type.  Of  these  magmas  albanose 
(III.  8.  2.  2)  is  the  most  common  and  most  widely  distributed,  occurring  at  various 
localities  all  around  the  lake,  though  mostly  to  the  south.  In  general  it  is  of  the 
romal  or  saccal  type,  but  the  melilitic  boval  albanose  occurs  in  the  neighborhood 
of  Montefiascone,  southeast  of  Lake  Bolsena.  The  only  other  subrangs  of  salfe- 
mane which  are  represented,  jugose  (III.  7.  2.  2)  and  fiasconose  (III.  7.  3.  2), 
are  rare  and  seem  to  be  confined  to  the  southeast  portion. 


162  THE  ROMAN  COMAGMATIC  REGION. 

The  magmas  of  the  Vulsinian  District  are  hence  highly  varied  and  the  number 
of  types  large.  Furthermore,  there  is  evident  a  progressive  change  in  composition 
from  northwest  to  southeast.  Thus  the  rocks  of  the  northern  and  northwestern 
portions  are  very  largely  either  persalane  or  dosalane,  the  former  apparently  the 
more  common;  those  of  the  Bolsena  and  Capodimonte  volcanoes  on  the  northeast 
and  southwest  of  the  lake  are  predominantly  in  the  dosalane  class;  while  to  the 
southeast,  around  the  Montefiascone  Volcano,  salfemane  rocks  are  the  largely 
predominant  ones;  and  furthermore,  it  is  only  in  this  portion  that  we  find  olivine 
and  melilite  entering  into  the  mode  to  a  notable  extent.  While  this  progression 
is  not  without  exceptions  and  irregularities,  and  although  my  knowledge  of  the 
northern  and  northwestern  parts  of  the  district  is  not  yet  satisfactorily  complete, 
the  observations  and  collections  which  I  was  able  to  make  seem  to  be  sufficiently 
ample  and  the  facts  observed  sufficiently  concordant  to  justify  the  assumption 
that  such  a  progressive  magmatic  change  does  occur.  Our  belief  in  the  validity  of 
this  supposition  is  confirmed  by  the  observation  of  similar  relations  in  other  districts. 

Of  the  various  subrangs,  ciminose,  vicose,  and  braccianose  are  probably  the 
most  abundant,  with  vulsinose  and  albanose  less  so,  though  still  common.  The 
other  subrangs  are  of  comparative  unimportance,  with  the  possible  exception  of 
vesuvose,  pulaskose,  and  procenose. 

CIMINIAN  DISTRICT. 

The  volcanic  structure  of  this  district  is  simpler  than  that  of  the  preceding 
one,  only  two  volcanoes  being  found,  and  these  clearly  distinct.  Petrographically, 
also,  the  character  is  less  complex,  the  number  of  subrangs  represented  being  only 
seven. 

Persalane  rocks  are  abundant,  though  only  two  subrangs  are  represented.  Of 
these  cumal  phlegrose  (I.  5.  i.  3)  [phonolitic  trachyte]  is  quite  common  in  blocks 
about  the  Vico  Volcano,  though  the  total  amount  is  probably  small.  On  the  other 
hand,  vulsinose  (I.  5.  2.  2)  is  very  common,  partly  of  the  non-leucitic  arsal  type 
as  flows  and  tuffs  about  the  Cimino  Volcano,  and  to  a  less  extent  around  the  Vico 
Volcano;  partly  of  the  pallanzanal  type,  known  locally  as  "peperino,"  with  small 
and  not  numerous  feldspar  and  leucite  phenocrysts  at  the  Cimino  Volcano;  and 
very  abundantly  at  the  Vico  Volcano  of  the  viterbal  type,  with  large  leucite  pheno- 
crysts, though  most  of  these  rocks  are  more  or  less  transitional  toward  ciminose. 

The  dosalane  rocks  vie  with  the  persalanes  in  point  of  abundance.  Heading 
the  list  systematically  is  harzose  (II.  4.  3.  3),  the  sorianal  type  of  which  is  so  common 
at  the  Cimino  Volcano.  The  occurrence  of  this  subrang  here  is  noteworthy  as 
the  only  quaric  subrang  found  along  the  whole  line  of  volcanoes  of  the  Roman 
Region,  if  we  except  the  Amiata  Volcano,  where  toscanose  (I.  4.  2.  3)  and  amiatose 

(I.  4-3-3)  occur- 

The  subrang  ciminose  (II.  5.  2.  2)  is,  with  vulsinose,  the  most  abundant  of 
the  district.  The  magnophyric  non-leucitic  arsal  type  [vulsinite]  forms  extensive 


PETROLOGY.  163 

flows  about  the  Cimino  Volcano.  Leucitic  types  of  ciminose  are  less  common. 
The  most  so  is  the  viterbal  type  [leucite-trachyte],  to  which  some  of  the  flows  of 
the  Vico  Volcano  doubtless  belong,  though  most  of  them  are  in  vulsinose.  The 
less  highly  porphyritic  bagnoreal  ciminose  [leucite-trachyte]  and  martinal  cimi- 
nose [leucite-tephrite]  are  rarer,  the  former  being  represented  only  in  the  Monte 
Venere,  the  last  eruption  of  Vico,  and  the  latter  by  some  flows  on  the  northwest. 
The  subrang  monzonose  (II.  5.  2.  3)  is  a  rather  rare  one,  and  forms  only  a  few 
flows  in  the  southern  portion  of  Vico.  On  the  other  hand,  the  subrang  of  vicose 
(II,  6.  2.  2)  is  abundant,  always  of  the  viterbal  type  [leucite-tephrite],  and  forms 
numerous  flows  of  the  Vico  Volcano. 

From  the  above  it  is  clear  that  the  older  Cimino  Volcano  to  the  northeast  is 
composed  almost  wholly  of  harzose,  vulsinose,  and  ciminose,  the  types  of  these 
being  non-leucitic  without  exception,  so  far  as  known.  The  Vico  Volcano  has, 
as  the  predominant  magmas,  vulsinose,  ciminose,  and  vicose,  with  some  phlegrose, 
and  small  quantities  of  monzonose,  leucitic  types  being  somewhat  predominant. 
As  a  whole,  then,  the  district  is  comparatively  simple  magma tically,  and,  as  we  shall 
see  later,  the  most  abundant  magmas  are  those — vulsinose,  ciminose  and  vicose — 
which  are  nearest  the  average  composition  of  the  whole  region.  It  is  especially 
noteworthy  that  none  of  the  subrangs,  as  braccianose  and  vesuvose,  belonging  to  the 
more  lenic  orders  are  found,  and  that  the  salfemanes  are  wholly  unrepresented. 

We  find  here  also  evidences  of  progressive  magmatic  change  in  the  rocks  of 
Cimino  and  Vico,  the  former  being  as  a  whole  considerably  more  salic  and  higher 
in  silica  than  those  of  Vico.  At  each  individually  there  does  not  seem  to  be  any 
definite  spatial  distribution,  a  condition  to  be  expected  when  it  is  considered  that 
in  each  case  there  was  apparently  but  one  outlet  for  the  flows. 

SABATINIAN  DISTRICT. 

This  district  resembles  the  Vulsinian  in  the  complexity  of  its  volcanic  struc- 
ture, as  well  as  in  the  range  of  its  subrangs,  though  magmatically  there  are  some 
striking  differences. 

The  persalane  class  is  comparatively  rare,  neither  phlegrose  nor  vulsinose, 
which  are  so  common  in  the  northern  districts,  being  met  with  here.  Of  this  class 
the  only  subrang  is  beemerose  (I.  6.  i.  3),  the  sabatinal  type  of  which  [leu cite 
phonolite]  occurs  in  the  northern  portion.  Of  dosalane  the  subrang  ciminose 
(II.  5.  2.  2)  is  wholly  unknown,  but  vicose  (II.  6.  2.  2)  is  rather  common  in  several 
types  [leucite-tephrite],  both  to  the  west  and  the  east  of  Lake  Bracciano.  Of  the 
subrangs  of  dosalane,  braccianose  (II.  7.  2.  2),  almost  always  of  the  galeral  type 
[leucitite],  is  by  far  the  most  common  and,  indeed,  the  most  characteristic  of  the 
district.  To  this  division  belong  nearly  all  of  the  extensive  flows  south  of  Lake 
Bracciano,  as  well  as  some  to  the  east  and  west.  The  subrang  vesuvose  (II.  8. 
2.  2)  is  also  rather  common,  likewise  of  the  galeral  type  [leucitite],  and  is  met  with 
in  the  northerly  parts  of  the  district. 


164 


THE    ROMAN  COMAGMATIC  REGION- 


The  salfemane  class  is  much  less  abundantly  represented,  the  only  subrang 
being  albanose  (III.  8.  2.  2),  of  the  romal,  saccal,  or  boval  types  [leucitite],  and 
is  met  with  at  several  points  about  the  lake,  both  north  and  south. 

It  is  seen  that  this  district  is  comparatively  simple  magmatically,  the  largely 
predominant  subrangs  being  braccianose  and  vicose,  with  relatively  small  amounts 
of  beemerose,  vesuvose,  and  albanose,  while  the  total  absence  of  such  widespread 
subrangs  as  vulsinose  and  ciminose  is  very  remarkable.  Another  important  char- 
acteristic is  the  fact  that  the  types  are  invariably  leucitic,  which  is  probably  connected 
with  the  uniformly  lenic  character  of  the  subrangs,  though  some  of  these,  as  beem- 
erose and  vicose,  assume  non-leucitic  modes  elsewhere. 

Although  the  more  northern  and  much  of  the  eastern  portions  of  this  district 
are  inadequately  known,  yet  the  general  distribution  of  magmas  described  above 
would  lead  us  to  suspect  that  here,  as  at  the  Vulsinian  District,  there  is  a  progressive 
magmatic  change  from  north  to  south,  the  femic  constituents  increasing  and  the 
silica  diminishing  in  this  direction. 

LATIAN  DISTRICT. 

This  small  district,  with  a  comparatively  simple  volcanic  structure,  is  even 
more  uniform  in  its  eruptive  products  than  the  one  just  described.  The  persalanes 
are  represented  only  by  the  very  rare  tavolatal  appianose  [haiiynitic  leucite- 
tephrite],  which  is  found  only  as  blocks  in  tuff.  Of  the  dosalanes  only  one  sub  rang 
can  be  surely  identified,  that  of  braccianose  (II.  7.  2.  2),  to  the  hernical  type  of 
which  [leucitite]  many  flows  can  be  referred.  The  salfemanes  are  probably  the 
most  abundant,  but  all  belong  to  one  subrang,  albanose  (III.  8.  2.  2),  mostly  of 
the  romal  and  saccal  types  [leucitite],  but  in  some  cases  of  the  melilitic  boval  type 
[cecilite]. 

The  district  is  thus  very  monotonous,  and  is  notable  for  the  highly  femic 
character  of  its  products  and  their  large  content  in  leucite,  as  well  as  for  the  total 
absence  of  non-leucitic  rocks. 

HERNICAN  DISTRICT. 

This  very  small  district  resembles  the  Latian  in  the  general  character  and 
uniformity  of  its  rocks.  None  belonging  to  persalane  occur,  nor  are  ciminose  or 
vicose  represented  among  the  dosalanes.  Of  the  rocks  of  this  class  the  majority 
fall  in  braccianose  (II.  7.  2.  2)  [leucite-tephrite],  with  a  few  probably  in  vesuvose 
(II.  8.  2.  2)  [leucitite]  and  one  probably,  though  doubtfully,  in  fiasconose  (II.  7. 
3.  2)  [leucite-basanite].  The  subrang  albanose  (III,  8,  2,  2)  of  salfemane  is  appar- 
ently as  common  as  braccianose,  the  rocks  being  of  the  saccal  type  [leucitite]. 

AURUNCAN  DISTRICT. 

As  our  knowledge  of  the  rocks  of  this  district  is  far  from  complete,  a  descrip- 
tion of  it  must  be  regarded  as  provisional  and  subject  to  future  corrections.  The 
persalane  rocks  seem  to  be  quite  common,  two  subrangs  being  represented, 


PETROLOGY.  165 

phlegrose  (I.  5.  i.  3),  of  both  the  ischial  and  cumal  types  [trachyte],  and  vulsinose 
(I.  5.  2.  2),  of  the  bolsenal  type  [vulsinite].  Apparently  the  majority  of  the  rocks 
belong  to  the  dosalane  class.  Of  this  the  subrang  ciminose  (II.  5.  2.  2)  is  rare, 
while  auruncose  (II.  5.  3.  2)  is  abundant,  two  types  [leucite-tephrite]  being  repre- 
sented, both  of  which  are  leucitic  and  one  carrying  considerable  biotite.  The 
subrang  shoshonose  (II.  5.  3.  3),  while  not  abundant,  is  important,  as  the  monfinal 
type  of  this  magma  [biotite- vulsinite]  forms  the  central  mass  of  Monte  Santa  Croce. 
Viterbal  vicose  (II.  6.  2.  2)  [leucite-tephrite],  with  abundant  large  leucite  pheno- 
crysts,  is  common,  probably  as  much  so  here  as  at  the  Ciminian  District.  While 
no  rocks  falling  in  braccianose,  vesuvose,  or  in  the  salfemane  class  were  collected 
by  myself,  it  would  seem  from  some  of  the  published  descriptions  that  such  do 
occur,  and  probably  rather  abundantly,  in  the  western  part  of  the  district.  These 
are  leucitic  and  presumably  of  types  found  elsewhere  [leucite-tephrite  and  leucitite]. 
In  addition  some  non-leucitic  types  [basalts]  are  described,  but  in  such  vague  terms 
that  nothing  definite  can  be  established  as  to  their  magmatic  character,  except 
that  they  are  almost  certainly  in  dosalane,  and  probably  in  the  perfelic  order 
germanare. 

From  these  meager  data  it  is  at  least  clear  that  the  rocks  of  the  Auruncan 
District  are  very  diverse  in  character,  the  magmatic  range  being  almost  as  great 
as  at  the  Vulsinian  District,  and  both  leucitic  and  non-leucitic  types  being  abundant. 
But  it  is  evident  that  much  more  study  is  needed  before  this  volcano  can  be  satis- 
factorily characterized. 

CAMPANIAN  DISTRICT. 

This  district  embraces  three  quite  distinct  centers  of  volcanic  activity;  the 
Vesbian  Volcano,  including  the  older  Somma  and  the  later  Vesuvius,  the  volcanic 
complex  of  the  Phlegrean  Fields,  and  the  island  of  Ischia,  which  is  essentially  the 
Epomeo  Volcano,  with  its  parasitic  cones.  The  rocks  of  Vesuvius  and  Somma 
are  remarkably  uniform,  most  of  them  falling  in  braccianose  (II.  7.  2.  2),  with 
fewer  in  jugose  (III.  7.  2.  2)  and  vesuvose  (III.  8.  2.  2),  several  different  types 
[leucite-tephrite]  being  represented.  The  Phlegrean  Fields  and  Ischia  are  simi- 
larly uniform,  though  here  the  character  of  the  magmas  is  quite  different.  In  these 
two  subdistricts  the  rocks  are  practically  all  in  persalane,  and  by  far  the  most 
predominant  subrang  is  phlegrose  (I.  5.  i.  3),  which  is  represented  by  several 
texturally  different  types  [trachyte].  Bolsenal  vulsinose  (I.  5.  2.  2),  pulaskose 
(I.  5.  2.  3),  and  monzonose  (II.  5.  2.  3)  [vulsinite]  are  found,  but  in  small  amounts 
comparatively,  and  some  leucitic  types  of  unknown  subrangs  occur,  but  always  as 
blocks  and  in  negligible  quantity. 

The  Campanian  District,  then,  is  composed  dominantly  of  but  two  subrangs, 
phlegrose  and  braccianose,  the  points  of  eruption  of  these  being  distinctly 
separated,  though  these  very  diverse  magmas  have  been  erupted  simultaneously 
at  times. 


i66  THE  ROMAN  COMAGMATIC  REGION. 

DISTRIBUTION  OF  MAGMAS. 

It  is  well  known  through  recent  researches  that  a  number  of  igneous  masses 
of  large  dimensions,  as  laccoliths,  show  a  very  pronounced  change  in  chemical 
and  other  characters  as  one  goes  from  the  center  to  the  periphery,  the  change  being 
sometimes  from  more  salic  to  more  femic,  and  sometimes  the  reverse.  These 
changes  are  usually  held  to  be  the  results  of  local  or  laccolithic  differentiation,  and 
typical  cases  have  been  discussed  by  Brogger,  Pirsson,  and  the  writer.  Similarly 
in  a  number  of  instances  there  have  been  observed  such  progressive  changes 
extending  over  petrographic  provinces  or  comagmatic  regions  of  large  area,  the 
most  detailed  being  that  of  central  Montana,  recently  described  by  Pirsson. 

Although  it  may  seem  at  first  glance  that  there  is  no  progressive  change  in  the 
rocks  of  the  Roman  Region,  but  that  the  different  districts  show  an  irregular 
alternation  of  magmatic  characters,  study  reveals  strong  evidence  of  a  regular 
progression  from  each  extremity  to  the  center.  In  considering  this  matter  some 
peculiarities  of  the  Roman  Region  must  be  borne  in  mind.  In  the  first  place,  it 
does  not  cover  an  area  extended  in  all  directions,  as  in  the  central  Montana,  Chris- 
tiania,  Bohemia,  and  other  well-known  regions,  but  rather  consists  of  a  long,  narrow 
line  or  band,  with  very  few  occurrences  of  igneous  rocks  on  either  side  of  the  main 
line,  and  those  very  small.  In  the  second  place,  the  occurrences  are  entirely  vol- 
canic, that  is,  effusive  and  superficial,  the  few  dikes  being  in  the  masses  of  volcanic 
cones,  and  hence  analogous  rather  to  lava  flows  so  far  as  their  bearing  on  problems 
of  differentiation  go,  deep-seated  dikes  penetrating  the  country  rock  and  plutonic 
or  deep-seated  masses  in  general  being  wholly  unknown.  A  third  consideration 
is  that  the  various  centers  are  strung  along  the  line  somewhat  irregularly  and  often 
with  very  notable  spaces  between  them. 

When  we  study  the  descriptions  of  the  different  districts  given  above  several 
points  are  brought  out. 

The  districts  at  the  extremities,  the  Vulsinian  and  the  Campanian,  are  the  most 
complex  vulcanologically,  those  inside  of  these,  the  Ciminian,  Sabatinian,  and 
Auruncan,  somewhat  simpler,  though  still  complex,  and  the  innermost,  the  Latian 
and  Hernican,  the  smallest  and  the  simplest  of  all. 

Concomitantly  with  this  progressive  change  in  vulcanological  structure,  though 
whether  causally  connected  or  not  need  not  be  here  considered,  is  a  more  or  less 
regular  change  from  ends  to  center  in  the  variety  of  the  magmas  represented.  Thus 
the  Vulsinian  District  shows  a  very  great  diversity  of  magmas,  varying  from  per- 
salic  to  salfemic  ones,  and  with  orders  running  from  perfelic  to  feldolenic,  while, 
on  the  other  hand,  the  rangs  are  consistently  domalkalic  and  the  subrangs  are 
mostly  dopotassic.  At  the  other  extremity  we  find  at  the  Campanian  District 
a  similarly  wide  diversity  of  magmas  from  perfelic  phlegrose  to  braccianose,  though 
here  the  number  is  not  as  great,  the  Vesbian  Volcano  being  almost  wholly  of 
braccianose.  while  the  Phlegrean  and  Epomean  Volcanoes  are  almost  entirely  of 
phlegrose,  near  the  border  of  nordmarkose.  The  penultimate  districts,  the  Cimin- 


PETROLOGY.  167 

ian  and  the  Auruncan,  are  almost  as  complex  and  great  in  range,  especially  the 
Auruncan,  while  the  Ciminian  is  distinctly  somewhat  simpler  and  with  a  smaller 
range,  the  more  lenic  orders  of  dosalane  and  the  salfemanes  not  being  found  here. 
The  three  central  districts,  the  Sabatinian,  the  Latian,  and  the  Hernican,  are  each 
much  more  simple  magmatically  than  the  external  pairs,  the  Sabatinian  being  the 
more  complex,  while  the  Latian  and  Hernican  both  show  only  a  very  limited  range 
of  magmatic  variation.  It  is  thus  seen  that,  while  certain  irregularities  are  evident, 
there  is  on  the  whole  a  marked  increase  in  magmatic  range  and  complexity  from 
the  center  to  the  extremities  of  the  line. 

Corresponding  with  this  variation  another  makes  itself  manifest,  though  the 
data  on  which  it  is  based  are  less  readily  and  satisfactorily  determinable.  If  we 
estimate  the  average  composition  of  the  magma  of  each  district  we  find  that  here 
also  there  is  a  distinct  and  definite  variation  between  center  and  ends.  This  esti- 
mation may  be  carried  out  in  two  ways,  either  by  weighting  the  different  chemical 
analyses  of  the  rocks  of  each  district  according  to  the  relative  amounts  and  taking 
the  mean,  or  more  roughly,  though  more  expeditiously,  by  expressing  the  average 
composition,  regard  being  had  to  the  relative  abundance  of  the  different  magmas 
at  each  center,  in  terms  of  the  magmatic  position  as  expressed  by  the  quantitative 
system  of  classification.  In  view  of  the  lack  of  data  and  imperfect  knowledge  of 
many  of  the  rocks  in  some  districts,  as  well  as  of  their  relative  amounts,  the  second 
method  would  seem  to  be  adequate  for  present  purposes. 

The  estimates  below  are  based  largely  on  my  own  observations,  as  well  as  on 
the  published  descriptions  of  others,  so  far  as  they  could  be  applied,  though  the 
details  can  not  be  gone  into  here,  and  while  the  results  are  admittedly  but  roughly 
approximate,  they  may  yet  be  considered  to  be  sufficiently  accurate  to  enable  us 
to  judge  the  main  features  with  confidence.  Of  these  estimates  those  for  the  Vul- 
sinian,  Sabatinian,  and  Auruncan  districts  are  the  least  reliable,  while  those  of  the 
others  are  based  on  fairly  complete  and  satisfactory  data. 

Average  Magmas  of  the  Districts. 

Vulsinian:    Vicose  (II.  6.  2.  2)  or  ciminose  (II.  5.  2.  2). 
Ciminian:     Ciminose  (II.  5.  2.  2)  or  vulsinose  (I.  5.  2.  2). 
Sabatinian:    Braccianose  (II.  7.  2.  2). 
Latian:    Braccianose  (II.  7.  2.  2)  or  vesuvose  (II.  8.  2.  2). 
Hernican:   Braccianose  (II.  7.  2.  2)  or  albanose  (III.  8.  2.  2). 
Auruncan:    Ciminose  (II.  5.  2.  2)  or  monzonose  (II.  5.  2.  3). 
Campanian:    Borolanose  (II.  6.  2.  3). 

Assuming  that  these  are  true  or  not  far  from  the  truth,  and  with  the  knowledge 
of  the  chemical  composition  and  norms  of  the  various  types  shown  by  the  numerous 
analyses  presented  on  a  previous  page,  it  is  clear  that  the  average  magmas  at  the 
ends  are  more  salic  and  less  lenic  and  femic,  that  is,  higher  in  silica  than  the  central 
ones,  which  are  very  distinctly  higher  in  femic  constituents  (especially  magnesia 
and  iron  oxides)  and  lower  in  silica. 


168  THE  ROMAN  COMAGMATIC  REGION. 

Expressed  mineralogically  and  in  terms  of  the  prevailing  systems  of  classifi- 
cation, we  find  toward  the  ends  of  the  line  an  abundance  of  non-leucitic  rock  types, 
trachytes,  vulsinites  and  ciminites,  with  leucite-trachytes,  leucite-phonolites,  leucite- 
tephrites,  and  some  leucitites;  while  at  the  central  districts,  the  Sabatinian,  Latian, 
and  Hernican,  we  find  no  non-leucitic  types,  that  leucite-trachytes  and  leucite- 
phonolites  are  rare,  and  leucite-tephrites  of  the  more  basic  types  and  leucitites  are 
largely  predominant. 

A  secondary  progression,  but  in  the  same  direction,  is  indicated  by  the  local 
distributions  observed  at  the  Vulsinian,  Ciminian,  and  Sabatinian  districts,  namely, 
that  in  each  the  southern  portion  is  more  femic  and  more  lenic  (less  high  in  silica) 
than  the  northern.  These  have  been  already  spoken  of  in  preceding  pages.  It 
is  needless  here  to  go  into  details,  but  there  is  little  doubt  that  as  the  volcanic 
foci  shifted  in  these  districts  there  was  a  progressive  variation  in  the  general 
magmatic  characters  of  the  erupted  rocks.  In  the  southern  half  of  the  region 
this  is  not  as  well  marked,  though  we  find  here  a  magmatic  division  of  the  districts. 
Thus  the  more  femic  magmas  seem  to  occupy  the  western  and  southwestern  parts 
of  the  Auruncan  District,  while  in  the  Campanian  the  more  salic  and  more  femic 
magmas  are  more  sharply  distinguished,  the  former  constituting  the  western  and 
the  latter  the  eastern  part. 

It  would  appear  also  that,  while  the  central  districts  are  very  preponderatingly 
dopotassic,  the  exterior  districts  are  less  so  and  tend  more  to  being  sodipotassic; 
that,  in  other  words,  there  is  a  progressive  increase  of  soda  relatively  to  potash 
from  the  center  outward.  This  is  shown  by  the  occurrence  of  rocks  belonging 
to  sodipotassic  subrangs,  as  phlegrose  (I.  5.  i.  3),  pulaskose  (I.  5.  2.  3),  procenose 
(I.  6.  2.  3),  harzose  (II.  4.  3.  3),  monzonose  (II.  5.  2.  3),  and  shoshonose  (II.  5. 
3.  3)  in  the  Vulsinian,  Ciminian,  Campanian,  and  Auruncan  districts,  while  rocks 
belonging  to  sodipotassic  subrangs  are  wholly  wanting,  or  found  only  sporadically 
in  the  three  central  ones.  The  fact  that  the  phlegrose  rocks  of  the  Campanian 
District  are  almost  dosodic  and  near  the  border  of  nordmarkose,  while  those  of 
the  Vico  Volcano  are  centrally  sodipotassic,  also  points  to  similar  relations. 

While  it  must  be,  and  indeed  freely  is,  admitted  that  the  data  on  which  the 
above  conclusions  rest  are  not  complete,  they  are  sufficiently  so,  in  the  judgment 
of  the  writer  and  in  view  of  the  descriptions  and  analyses  found  in  the  preceding 
pages,  to  warrant  belief  in  a  regular  and  symmetrical  distribution  of  the  magmas 
in  the  Roman  Region,  in  which  the  central  magmas  are  not  only  less  varied  and 
complex,  but  are  more  femic  and  lenic,  lower  in  silica,  and  with  potash  relatively 
much  higher  than  soda,  than  obtains  in  the  districts  toward  both  extremes  of  the  line. 

Although  anticipating  a  point  which  will  be  brought  out  later,  it  will  be  of 
interest  to  note  here  that  the  average  composition  of  the  regional  magma  as  a  whole 
is  probably  a  ciminose-vicose  (II.  5-6.  2.  2);  that  is  to  say,  that  the  average  exterior 
magmas  are  of  about  the  composition  of  the  original,  or  proximately  original,  magma, 
while  the  central  ones  show  compositions  approaching  the  more  femic  extremes 
of  differentiation. 


PETROLOGY.  169 

PROGRESSION  OF  TYPES. 

In  a  recent  paper*  Pirsson  calls  attention  to  a  phase  of  the  occurrence  of  the 
rock  types  in  Central  Montana  Region,  to  which  he  applies  the  name  of  "Regional 
Progression  of  Types. "  The  idea  to  be  expressed  by  this  term  is  that  in  traversing 
a  given  comagmatic  region  certain  types,  from  being  sporadic,  become  frequent  and 
then  disappear,  the  rare  types  of  one  district  or  portion  of  a  region  becoming  the 
common  ones  of  other  portions.  As  he  says  that  "it  would  be  a  matter  of  interest 
to  know  if  this  progression  of  types  is  a  peculiarity  confined  to  this  (Central  Mon- 
tana) province  and  occasioned  by  the  local  distribution  of  magmas,  or  whether  it 
is  of  more  general  application,"  and  as  he  suggests  the  region  of  central  Italy  as 
one  in  which  it  is  likely  to  be  observed,  it  will  be  well  to  examine  our  region  briefly 
from  this  point  of  view. 

It  may  be  stated  at  the  outset  that  a  similar  progression  does  seem  to  exist, 
though  not  as  clearly  cut  in  many  ways,  as  seems  to  be  the  case  in  the  Montana 
one.  This  is  possibly  due  to  the  fact  that  in  the  Italian  region  the  rocks  are 
entirely  extrusive.  As  instances  of  the  kind  the  following  may  be  mentioned : 

The  galeral  and  hernical  types  of  braccianose  and  the  romal  and  saccal  of 
albanose  [leucitites]  occur  in  abundance,  and  form  the  majority  of  the  flows  in  the 
Hernican  and  Latian  and  the  southern  part  of  the  Sabatinian  Districts.  To  the 
south  of  the  first  of  these,  these  types  are  rather  common,  though  somewhat  sporadic, 
in  the  Auruncan  District,  but  are  quite  absent  in  the  Campanian,  the  types  of  brac- 
cianose here  being  quite  distinct.  To  the  north  these  types  are  unknown  in  the 
Ciminian  District,  but  reappear  in  the  Vulsinian,  especially  in  the  southern  part, 
to  die  out  in  the  northern. 

The  ischial  and  cumal  types  of  phlegrose  [augite-trachyte]  are  most  abundant 
in  the  western  part  of  the  Campanian  District,  they  (especially  the  cumal)  are 
rather  abundant  but  sporadic  in  the  Auruncan  District,  disappear  in  the  central 
ones,  to  recur  as  the  scattered  blocks  of  cumal  phlegrose  in  the  Ciminian.  Simi- 
larly, but  in  a  reversed  direction,  the  arsal  types  of  vulsinose  and  monzonose  (vul- 
sinite)  are  fairly  common  in  the  northerly  Vulsinian  District,  attain  a  maximum  in 
the  Ciminian,  are  lost  in  the  three  central  districts,  occur  in  the  Auruncan,  and  are 
very  rare  in  the  Campanian. 

The  types  with  the  characteristic  viterboid  habit,  viterbal  vulsinose,  ciminose, 
and  vicose  [leucite-trachyte],  show  a  closely  similar  distribution.  Occurring  rarely 
in  the  Vulsinian  District,  they  form  the  most  prominent  rocks  of  the  Vico 
Volcano  in  the  Ciminian,  vanish  in  the  central  districts,  and  are  rather  common 
in  the  Auruncan,  though  they  do  not  reappear  in  the  Campanian. 

These  are  the  most  prominent  examples,  though  others  might  be  given  or  can 
be  found  by  the  reader  by  examination  of  the  descriptions  of  the  types,  and  they 
show  clearly  that  a  progression  of  types  does  exist  in  the  Roman  Region,  and  that 
such  a  phenomenon  is  not  local  and  a  peculiarity  of  the  Central  Montana  Region, 

*  L.  V.  Pirsson,  Am.  Jour.  Set.,  XX,  1905,  p.  48. 


1 7°  THE  ROMAN  COMAGMATIC  REGION. 

but  one  which  is  widespread  and  probably  to  be  observed  elsewhere,  now  that 
attention  has  been  directed  to  it. 

THE  ROCKS  OF  THE  TUSCAN  REGION. 

In  this  connection  a  few  words  may  be  devoted  to  the  relations  of  the  rocks 
of  the  volcanoes  making  up  the  Tuscan  Region  to  those  considered  in  this  paper. 
It  has  been  mentioned  that  these  volcanoes  are  all  of  rather  small  size  and  area, 
their  eruptions  being  apparently  of  the  domal  type,  and  that  they  lie  close  to  the 
area  of  the  Roman,  but  exterior  to  it,  on  the  north  and  west. 

It  is  unnecessary  here  to  go  into  descriptions  of  the  rocks,  which  will  be  found 
in  several  publications  mentioned  in  the  bibliography,  especially  in  Nos.  4,  15,  43, 
62,  and  65.  It  must  suffice  to  say  that  the  rocks  are  quite  uniform  in  character, 
mostly  toscanites  or  quartz-trachyandesites  in  the  prevailing  nomenclature,  while 
in  the  quantitative  classification  they  fall  in  dellenose  (I.  4.  2.  2),  toscanose  (I.  4. 
2.  3),  or  amiatose  (I.  4.  3.  3).* 

This  general  uniformity,  or  rather  comparatively  restricted  range  in  chemical 
composition,  their  persalic  and  quardofelic  characters,  and  the  prevalence  of  sodi- 
potassic  subrangs  are  features  which  fall  in  line  with  the  general  course  of  the  dis- 
tribution of  magmas  in  the  Roman  Region,  where  we  have  just  noted  the  more 
salic,  felic,  and  sodipotassic  characters  of  the  exterior  portions  as  contrasted  with 
the  central  ones.  It  suggests  the  thought  that  these  Tuscan  rocks  may  belong  to 
the  same  general  body  of  magmas  as  those  of  the  Roman  Region,  and  that  they 
represent  in  fact  the  extreme  exterior  differentiation  products. 

While  this  hypothetical  magmatic  connection  is  very  tempting,  it  must  be 
noted  that  a  serious  objection  to  it  lies  in  the  fact  that  the  volcanoes  of  the  Tuscan 
Region  are  apparently  all  of  about  Eocene  age,  while  those  of  the  Roman  Region 
are  much  later,  belonging  to  the  Pliocene  or  even  modern  times. 

Whether  this  objection  on  the  score  of  time  relations  is  a  valid  one  or  not,  we 
are  scarcely  in  a  position  to  judge  as  yet,  though  consideration  of  the  very  great 
time  intervals  involved  would  seem  to  render  the  objection  to  the  hypothesis  of 
more  weight  than  the  chemical  and  petrological  evidence  in  its  favor.  At  the  same 
time  it  may  be  mentioned  that  the  actual  time  interval  between  the  earliest  and 
the  latest  of  the  eruptions  in  the  Roman  Region,  which  are  undoubtedly  connected 
magmatically,  is  itself  very  great,  so  that  possibly  the  objection  raised  here  is  not 
of  such  a  serious  character  as  might  appear  at  first  sight.  At  any  rate,  the  hypoth- 
esis here  presented  may  be  borne  in  mind  in  further  studies  of  the  Italian  volcanic 
rocks. 

Quantitative  Relations. 

In  any  comagmatic  region  where  the  igneous  rocks  are  all  effusive  it  is  in  a 
high  degree  improbable  that  they  represent,  in  the  aggregate,  the  composition  of 
the  original  magma  from  which  they  have  been  derived.  This  is  so  since,  assuming 

*  Cf.  Washington,  Prof.  Paper,  U.  S.  Geol.  Surv.    No.  14,  1903,  pp.  159,  171,  185. 


PETROLOGY.  171 

the  existence  of  some  deep-seated  "reservoir,"  "magma-basin,"  or  other  body 
of  magma  as  a  source,  it  is  highly  improbable  that  these  extruded  portions  form 
more  than  a  small  fraction  of  the  whole,  or  that  their  relative  proportions  when 
averaged  are  such  as  to  give  a  correct  idea  of  the  true  composition  of  the  magma 
whence  they  are  derived.  It  is  probable  that  the  body  of  extruded  rock  is  very 
small  relatively  to  the  parent  body  and  that  it  consists  for  the  most  part  of  only  a 
certain,  and  not  wholly  representative,  portion  of  the  differentiates. 

The  above  conclusions  would  also  hold  good,  though  probably  to  a  less  extent, 
of  regions  where  the  erosion  has  been  profound,  so  that  the  deep-seated  rock  bodies, 
as  stocks  and  laccoliths,  are  exposed,  with  their  satellites  of  dikes  and  sometimes 
of  flows.  In  most  cases  it  is  probably  true  that  here  erosion  has  destroyed  a 
very  considerable  portion  of  the  masses,  especially  of  the  effusive  portions,  while 
again  it  has  not  progressed  far  enough  to  reveal  the  deeper  portions,  and  we  are 
left  in  the  dark  as  to  the  downward  extensions. 

In  both  cases  the  practical  difficulty  of  estimating  the  relative  masses  of  the 
various  types  is  a  serious  one.  In  certain  small  bodies,  as  some  differentiated  dikes 
or  small  and  deeply  eroded  laccoliths,  as  that  of  Shonkin  Sag,  exceptionally  favor- 
able conditions  may  almost  entirely  overcome  the  inherent  difficulties,  and  very 
reliable  estimates  can  be  made.  But  in  the  great  majority  of  cases  the  difficulties 
are  almost  or  quite  insuperable,  and  we  must  perforce  remain  content  with  such 
approximate  estimates  as  the  erosion  and  exposures  permit. 

But  notwithstanding  the  impossibility  in  most  cases  of  arriving  at  perfectly 
satisfactory  or  trustworthy  data,  the  problem  of  the  composition  of  the  original 
magma  is  of  such  interest  in  the  study  of  comagmatic  regions,  and  has  such  impor- 
tant bearings  on  the  phenomena  of  differentiation,  that  an  attempt  should  be  made 
to  determine  it  wherever  possible,  even  though  the  results  arrived  at  must  be 
regarded  as  of  no  very  high  order  of  accuracy  and  provisional  in  their  character. 
As  such  observations  increase  in  number  and  cover  more  kinds  of  comagmatic 
regions  or  differentiated  bodies,  their  value  will  increase,  even  though  they  be  not 
absolutely  accurate,  since  in  any  case  they  will  probably  point  out  the  directions 
in  which  the  processes  of  differentiation  take  place,  if  not  their  extent.  We 
shall  at  least  gain  some  general  knowledge,  though  all  the  details  are  unknown 
to  us. 

In  the  case  of  the  Roman  Region,  as  much  of  it  is  as  yet  unmapped  geologically, 
and  in  any  case  the  thicknesses  of  the  various  flows  are  quite  unknown  for  the  most 
part,  as  well  as  many  of  them  certainly  covered  beyond  our  observation  by  later 
flows  and  tuffs,  the  possibility  of  detailed  knowledge  of  relative  masses  is  reduced 
almost  to  a  minimum.  The  only  basis  on  which  I  can  estimate  them  at  present 
are  my  general  observations,  and  the  published  statements  of  others,  as  to  the 
relative  abundance  of  the  various  types  which  are  accessible  to  study,  the  relative 
areas  of  the  different  districts  being  also  taken  into  consideration.  We  shall  begin, 
therefore,  with  a  general  examination  of  the  quantitative  relations  of  the  different 


172  THE  ROMAN  COMAGMATIC  REGION. 

magmas,  the  spatial  relations  of  the  different  magmas  and  types  being  discussed 
elsewhere,  and  shall  then  attempt   to  estimate  the  average  composition  of  the 

whole. 

QUANTITATIVE  RELATIONS  OF  THE  MAGMAS. 

The  persalanes,  while  widely  distributed,  are  not  the  most  abundant  rocks, 
forming  possibly  but  one-quarter  or  one-third  of  the  whole.  The  great  majority 
belong  to  the  perfelic  order  canadare,  rocks  of  the  lenic  orders  russare  and  tas- 
manare  being  rare.  For  the  most  part  they  are  domalkalic,  either  vulsinose  or 
pulaskose,  the  former  subrang  being  the  more  abundant,  with  the  peralkalic  and 
sodipotassic  phlegrose  in  greater  amount  than  pulaskose  but  less  than  vulsinose. 

The  dosalanes  constitute  the  greater  part  of  the  rocks,  one  might  say  rather 
more  than  one-half.  Of  these  those  belonging  to  the  quaric  order  austrare  are 
very  few,  the  perfelic  germanares  are  very  abundant,  and  probably  in  somewhat 
greater  amount  than  these  last  are  collectively  the  lenic  norgares,  italares,  and  cam- 
panares.  By  far  the  most  of  the  rocks  which  fall  in  this  class  are  domalkalic,  with 
fewer  alkalicalcic  and  no  peralkalic  or  docalcic  ones.  The  subrangs  are  generally 
dopotassic,  though  sodipotassic  subrangs  are  not  unusual,  but  are  generally  con- 
fined to  the  less  common  types.  Of  the  subrangs  of  dosalane  it  may  be  said  that 
ciminose,  braccianose,  and  vicose  are  most  abundant,  with  less  amounts  of  mon- 
zonose  and  vesuvose,  the  others  being  comparatively  rare. 

The  salfemanes  are  less  abundantly  represented  than  either  of  the  other  two 
classes,  and  probably  considerably  less  than  a  quarter,  let  us  say  one-fifth,  of  the 
rocks  belong  here.  With  few  exceptions  the  salfemane  rocks  are  in  the  lendofelic 
order  of  bohemare,  and  of  the  subrangs  of  salfemane  that  of  albanose  is  undoubtedly 
the  most  abundant. 

Summing  up  the  above,  we  may  make  the  following  roughly  quantitative 
statement,  in  which  the  relative  abundance  of  the  subrangs,  in  the  judgment  of 
the  writer,  is  that  of  the  order  in  which  they  appear.  It  would  be  impossible  to 
give  more  exact  quantitative  data  without  very  minute  and  detailed  study  of  all 
the  districts. 

Order  of  Abundance  of  Subrangs. 
Abundant. — Ciminose    (II.  5.  2.  2),    braccianose    (II.  7.  2.  2),    vicose    (II.  6.  2.  2),    vulsinose 

(I-  5-   2-  2). 

Common. — Albanose  (III.  8.2.2),  phlegrose  (1.5. 1.3),  monzonose  (II.  5.  2.  3),  vesuvose  (II.  8.2.2). 
Rare. — Pulaskose  (I.  5.  2.  3),  auruncose  (II.  5.  3.  2),  harzose  (II.  4.  3.  3),   shoshonose   (II,  5. 

3-  3)>  jugose  (III.  7.  2.  2),  beemerose  (I.  6.  i.  3). 
Very  rare. — Procenose  (I.  6.  2.  3),  fiasconose  (III.  7.  3.  2),  appianose  (I.  7.  i.  3). 

The  reliability  of  these  relative  positions  varies  much.  Thus  the  first  four 
are  undoubtedly  easily  the  most  abundant,  but  the  orders  of  ciminose  and  brac- 
cianose, and  of  vicose  and  vulsinose,  are  open  to  question.  The  same  is  true  of  the 
"common"  magmas,  which  collectively  should  follow  the  "abundant"  ones,  though 
there  may  be  some  question  as  to  the  relative  abundance  of  albanose  and  phlegrose, 


PETROLOGY.  173 

or  of  monzonose  and  vesuvose.  Similar  criticisms  could  be  made  of  the  rare  and 
very  rare  magmas,  though  the  positions  of  the  last  two  are  almost  certain,  but  all 
of  these  are  so  exceptional  that  they  have  little  bearing  on  the  immediate  problem. 

THE  AVERAGE  MAGMA. 

As  we  have  seen  that  the  relative  amounts  of  the  different  subrangs  vary  greatly, 
from  those  which  are  very  abundant  to  those  which  are  only  met  with  excep- 
tionally, it  is  clear  that  the  composition  of  the  whole,  or  of  the  original  magma, 
can  not  be  arrived  at  by  simply  taking  the  average  of  all  the  analyses.  This  would 
lead  to  correct  results  if  the  number  of  analyses  of  each  type  were  proportional  to 
its  relative  abundance  or  less  accurately  to  the  number  of  occurrences.  But  as 
this  is  not  true  we  must,  in  our  final  calculation,  weight  the  analyses  of  the  rocks 
belonging  to  each  type  or  subrang  according  to  the  relative  abundance  as  best 
we  may.  This  is  a  somewhat  difficult  proceeding,  and  in  the  nature  of  the  case 
and  the  absence  of  exact  quantitative  data,  must  be  somewhat  arbitrary  and  depend 
largely  on  the  judgment  of  the  calculator.  The  matter  has  been  the  subject  of 
considerable  thought  and  numerous  calculations,  the  latter  indicating  that  the 
range  of  possible  variation  in  different  directions  was  much  smaller  than  had  been 
thought  probable,  and  it  was  also  found  that  the  influence  of  small  amounts  of 
the  rarer  magmas  was  almost,  if  not  quite,  negligible. 

As  a  final  basis  of  calculation  it  was  decided  to  employ  only  the  more  abundant 
magmas,  the  relative  amounts  of  which  could  be  approximately  estimated  with  a 
fair  degree  of  satisfaction.  The  introduction  of  the  rarer  ones  into  the  calculation 
must  be  regarded  as  a  small  correction,  to  be  applied  in  the  future,  when  the  main 
data  are  more  accurately  known.  To  a  certain  extent  also  these  rarer  magmas  are 
of  such  characters  that  they  compensate  for  each  other  more  or  less,  so  that  their 
introduction  or  neglect  does  not  seem  to  affect  the  final  result  seriously. 

The  magmas  selected  for  use  were  ciminose  (II.  5.  2.  2),  braccianose  (II.  7. 
2.  2),  vicose  (II.  6.  2.  2),  vulsinose  (II.  5.  2.  2),  albanose  (III.  8.  2.  2),  phlegrose 
(I.  5.  i.  3),  and  monzonose  (II.  5.  2.  3).  The  average  of  the  analyses  of  the  rocks 
of  the  various  types  which  fall  in  each  of  these*  was  calculated,  and  each  average 
was  weighted  as  follows:  Ciminose,  braccianose,  and  vicose = weight  5;  vulsinose  = 
weight  4;  albanose,  phlegrose,  and  monzonose  =  weight  2. 

It  would  have  been  well  to  include  vesuvose,  with  a  weight  of  2,  but  no  chemical 
analysis  of  this  was  available,  and  it  is  probable  that  its  neglect  is  compensated  for  by 
the  possible  greater  abundance  of  phlegrose  over  albanose.  Also,  it  might  have 
been  somewhat  more  accurate  to  include  pulaskose,  auruncose,  and  harzose  with 
a  weighting  of  i  each,  but  it  was  thought  that  this  was  giving  these  rather  rare 
magmas  undue  importance,  and  it  was  found  that  a  less  relative  weighting  had  but 
little  effect  on  the  final  result. 

In  any  accurate  estimate  corrections  for  the  specific  gravities  of  the  different 
types  should  also  be  made.  But  the  data  in  regard  to  the  relative  abundance 
are  so  rough  in  themselves  that  this  was  deemed  to  be  an  unnecessary  refinement. 

*  Except  that  of  the  viterbal  vulsinose  from  below  San  Rocco,  at  the  Vico  Volcano  (cf.  p.  39). 


174 


THE  ROMAN  COMAGMATIC  REGION. 


Calculating  the  average  on  the  bases  given  above,  and  neglecting  water  and 
the  minor  constituents  except  TiO2  and  P2OS,  the  following  results  were  obtained: 

Composition  of  the  Average  Magma. 

SiOa 53.42  0.890 

A12O3 18.29  .179 

FeaO3 2.94  .018 

FeO 3 . 44  . 048 

MgO 3 . 10  . 078 

CaO 6.37  .114 

Na2O 2 . 96  . 048 

KaO 8.31  .088 

TiOa 0.82  .010 

P»OS 0.35  .002 

99-97 


Norm  of  the  ; 
Or  /i8 

iverage  Mai 

93) 
72  £65.60} 

95) 
08     11.08} 

60  ) 

18  {    5-6o 

52  )    - 

%ma. 
•76.68 

23.29 

Ab  

.    4 

An  

II 

Ne  

II 

Di  

.  .  14 

Ol  

2 

Mt  

11  

I 

Ap  

According  to  this  calculation  the  average  or  original  magma  falls  almost 
exactly  at  the  center  point  of  dosalane,  is  fairly  well  within  the  lendofelic  order  nor- 
gare,  though  somewhat  close  to  the  border  of  the  perfelic  germanare,  is  almost 
exactly  at  the  center  point  of  the  domalkalic  rang  essexase,  and  is  well  within  the 
borders  of  the  dopotassic  subrang  vicose.  It  is  thus  a  vicose  (II.  6.  2.  2),  somewhat 
approaching  a  ciminose  (II.  5.  2.  2),  though  not  sufficiently  closely  to  be  called  a 
ciminose-vicose.  Its  general  composition  is  therefore  that  of  two  of  the  most  abun- 
dant subrangs,  and  it  is  clear  that  the  influences  of  the  others  which  were  introduced 
into  the  calculation  have  largely  counterbalanced  each  other,  the  persalanes  the 
salfemanes,  the  perfelic  orders  those  which  are  more  lenic,  and  the  peralkalic  rangs 
those  which  are  alkalicalcic,  while  the  sodipotassic  subrangs  reduce  slightly  the 
markedly  dopotassic  character  of  the  others. 

None  of  the  rocks  which  were  analyzed  have  exactly  the  same  composition, 
though  the  analyses  of  bagnoreal  ciminose  and  viterbal  vicose  most  resemble  it  in 
general  features.  In  the  former,  however,  the  silica  is  a  little  higher  (the  order 
being  perfelic  on  this  account),  and  the  bivalent  oxides  are  lower,  while  in  the 
vicose  analyses  alumina  is  a  little  higher  and  potash  considerably  so. 

When  the  position  of  this  average  analysis  is  referred  to  the  diagram  given 
farther  on  (p.  184),  it  is  seen  that  it  falls  almost  exactly  in  the  middle  of  the  gap  be- 


tween  SiOj  =  52  .40,  and  the  critical  group,  with  a  ratio  of  ^r-^=  10.  u,  or  almost 

JV2U 

exactly  the  average  one.  It  would  seem  from  these  facts  that  the  composition  as 
calculated  must  approximate  very  closely  to  the  true  average  of  the  effusive  rocks 
so  far  as  they  are  known. 

It  is  worthy  of  remark  that  rocks  belonging  to  vicose  and  ciminose,  or  which 
approach  the  average  in  chemical  composition,  occur  only  in  those  districts  where 
there  is  the  greatest  variety  of  magmas,  namely  the  Vulsinian,  Ciminian,  Saba- 
tinian,  and  Auruncan,  and  which,  furthermore,  we  have  reason  to  believe  are  the 
oldest.  They  seem  to  be  entirely  wanting  in  those  districts  where  the  rocks  are 
more  uniform  in  character,  the  Latian  and  the  Hernican,  as  well  as  in  the  Cam- 


PETROLOGY.  175 

panian  District  with  its  two  subdistricts,  each  of  very  uniform  rocks,  the  one  highly 
salic  and  perfelic,  and  the  other  more  lenic  and  more  femic  than  the  average. 

We  have  also  seen  in  the  discussion  of  the  space  relations  of  the  types  and 
magmas  that  the  average  compositions  of  the  outer  districts,  the  Vulsinian,  Ciminian, 
Auruncan,  and  Campanian,  are  probably  very  close  to  that  of  the  average  magma 
of  the  region  as  a  whole.  In  the  first  three  of  these  districts  the  relations  are  too 
complex  and  the  number  of  types  too  great  to  allow  of  very  satisfactory  estimates. 
But  in  the  comparatively  simple  Campanian  District  there  seems  to  be  a  possibility 
of  testing  this  statement  by  calculation.  For  this  district  the  average  has  been 
arrived  at  by  taking  the  average  of  the  three  analyses  of  Vesuvian  lavas  given  in 
the  table  to  find  the  composition  of  the  Vesuvian  magma.  That  of  the  Phlegrean 
and  Ischian  magma  was  found  by  taking  the  average  of  the  analyses  of  the  phleg- 
rose  rocks  of  the  district,  and  of  the  vulsinose  and  monzonose  rocks  of  Astroni  and 
L'Arso,  and  weighting  these  in  the  proportion  of  5  to  i,  as  the  phlegrose  rocks  are 
much  the  most  abundant.  These  two  averages  were  then  averaged  by  taking 
equal  parts,  with  the  results  here  given. 


77-29 


A' 
COMPOSITION. 
SiOa  e^ 

uera 

.0 
.2 

6 
9 
•  7 

6 

3 
5 
8 

4 

ge  Camj. 

0.900 
.178 
.016 
•  054 
.068 
.100 
.069 
.080 
.010 
.003 

Banian  Magma. 
Noi 
Or  AA 

M. 

48) 

26  £ 
06) 

49 
66  \ 

71  I 

52$ 

96 

63 

13 
16 

5 

0 

80  ) 

49  J 

23 
96 

A1..O, 

X8 

Ab  

ii 

Fe,O» 

2 

An  

8 

FeO  

.     -I 

Ne  

.  .  17 

MeO  .  . 

2 

Di  

CaO     

.    e 

Ol  

2 

NaaO        

.    4. 

Mt  

.     \ 

K2O 

.    7 

11  

I 

TiO2  

O 

Ap  .. 

O 

P.CK 

on 

80 

22.51 


The  position  of  this  magma  is  in  borolanose  (II.  6.  2.  3).  The  general  resem- 
blance to  the  average  magma  of  the  whole  region  is  very  close,  both  chemically 
and  normatively,  the  only  difference  of  importance  being  in  the  relations  of  potash 
to  soda,  which  is  here  almost  unity,  potash  being  only  very  slightly  higher  than  the 
soda.  This  is,  of  course,  due  to  the  peculiar  character  of  the  phlegrose  rocks  of 
the  district,  which  are  so  high  in  soda  as  to  be  almost  in  nordmarkose  (I.  5.  i.  4). 

Time  Relations. 

GEOLOGIC  AGE  OF  THE  ERUPTIONS. 

It  would  lead  us  too  far  astray,  and  would  be  outside  of  the  province  of  this 
paper,  to  discuss  this  topic  fully  and  adequately,  so  a  few  words  must  suffice.  Fur- 
thermore, there  exists  considerable  divergence  of  opinion  in  regard  to  the  exact 
age  of  the  eruptions  in  some  cases. 

While  the  volcanoes  of  the  Tuscan  Region  date  from  the  Eocene  or  Miocene 
Period,  those  of  the  Roman  Region  have  begun  either  in  the  late  Pliocene  or  in  the 


176  THE  ROMAN  COMAGMATIC  REGION. 

Pleistocene  (Quaternary),  and  in  some  cases  have  continued  into  the  present  ti 
There  does  not  seem  to  be  any  good  evidence  in  favor  of  a  regular  shifting  of  th 
volcanic  foci  along  the  line  in  any  direction,  that  is,  that  the  volcanic  action  began 
successively  later  either  northwardly  or  southwardly.  On  the  contrary  it  is  quite 
clear  that  some  of  them  have  been  active  simultaneously,  or  at  least  that  the  periods 
of  activity  have  overlapped. 

But  there  seems  to  be  no  doubt  of  the  fact  that  the  extinction  of  activity  has 
shifted  southerly,  and  that  the  more  northern  volcanoes  were  extinct  before  the 
more  southern  ones.  That  this  extinction  was  successive  in  a  general  way,  though 
possibly  with  some  irregularities,  seems  also  to  be  true.  Thus  the  Vulsinian  vol- 
canoes, which  began  before  those  of  the  Campanian  District  according  to  Moderni, 
the  Ciminian  and  the  Sabatinian  have  all  suffered  much  from  erosion,  so  much 
so  as  to  have  lost  much  of  their  original  forms.  The  Latian  Volcano  seems  to  have 
been  active  in  the  year  639  B.  c.  (114  A.  IT.  c.),  if  we  can  believe  the  story  of  Livy* 
as  to  a  shower  of  ashes  from  Mount  Albanus.  The  age  of  the  Hernican  volcanoes 
is  uncertain,  but  they  were  probably  extinct  before  the  Latian,  as  we  have  no  records 
of  an  eruption  in  historic  times.  There  is  some  evidence  going  to  show  that  the 
Auruncan  Volcano  was  active  during  Roman  times,  and  the  date  269  B.  c.  has 
been  assigned  to  one  eruption.  But  the  evidence  is  not  conclusive,  and  the  general 
silence  on  the  part  of  the  Latin  authors  makes  one  somewhat  skeptical  as  to  so 
recent  a  date.f  The  modern  activity  of  Vesuvius  is  too  well  known  to  need  more 
than  mention,  while  the  last-recorded  eruption  of  Ischia  was  in  1302,  and  that  of 
the  Phlegrean  Fields  in  1538  at  Monte  Nuovo,  although  the  Solfatara  Volcano  is 
still  emitting  hot  vapors. 

ORDER  OF  SUCCESSION  OF  TYPES. 

A  much  more  detailed  study  of  the  various  districts  than  the  writer  was  able 
to  give  and  an  extensive  collation  of  the  literature  are  needed  before  this  matter 
can  be  satisfactorily  treated.  From  such  observations  as  were  made,  however, 
and  from  a  general  survey  of  the  literature,  it  would  appear  that  no  definite  and 
constant  order  of  succession  obtains  in  the  region.  Salic  types  sometimes  precede 
and  sometimes  follow  the  more  femic  ones,  those  with  leucite  sometimes  underlie 
and  sometimes  overlie  those  without  leucite.  Instances  of  both  kinds  could  be 
cited  from  the  more  complex  districts,  the  Vulsinian,  Ciminian,  Sabatinian,  and 
Auruncan.  An  order  which  seems  to  hold  good  in  general,  though  with  exceptions, 
in  one  district  is  contradicted  by  the  facts  of  another.  Indeed,  so  varied  are  the 
relations  in  this  respect  that  the  only  general  law  which  can  be  laid  down  for  the 
region,  at  least  for  the  present,  is  that  there  is  no  definite  order  of  succession. 

*  Cf.  V.  Sabatini,  Mem.  Dtscr.  Carta  Geol.  Ital.,  X,  1900,  p.  132. 
t  P.  M  oderni,  Boll.  Com.  Geol.  Ital.,  1887,  p.  77. 


COMPARISON  WITH  OTHER  REGIONS. 

It  is  only  within  the  last  few  years,  and  following  the  increase  in  the  number 
and  quality  of  chemical  analyses  of  igneous  rocks,  that  comagmatic  regions  (petro- 
graphic  provinces)  have  been  described  in  terms  that  permit  of  satisfactory  corre- 
lation between  them,  with  a  view  to  their  bearing  on  the  phenomena  and  processes 
of  differentiation.  And  even  now  the  number  of  those  which  have  been  at  all 
adequately  described  is  so  small  that  it  would  be  unwise  to  attempt  any  broad  gen- 
eralizations. These  must  wait  for  the  future  and  a  great  increase  in  our  detailed 
knowledge,  especially  as  regards  the  chemical  side,  of  many  widely  diverse  regions. 
At  the  same  time  it  may  be  profitable,  and  not  without  interest,  to  institute  com- 
parisons with  one  or  two  others  of  the  best-known  regions.  The  phenomena  are 
so  complex  that  even  tentative  comparisons  may  be  of  use  in  pointing  out  some  of 
the  lines  along  which  it  will  be  most  profitable  to  work,  or  in  calling  attention  to 
certain  lacunae  in  our  knowledge  or  to  features  which  have  been  overlooked  and 
which  may  demand  investigation. 

From  a  general  survey  of  the  literature  it  is  clear  that  it  is  far  easier  to  find 
well-described  regions  which  differ  radically  from  the  Roman  than  to  find  those 
which  resemble  it.  Thus  the  Christiania  Region,  so  well-known  through  the  classic 
researches  of  Brogger,  that  of  Madagascar,  recently  described  by  Lacroix,  and  that 
of  eastern  Africa,  known  through  the  studies  of  Prior  and  others,  while  they  resemble 
the  Roman  in  the  dominantly  alkalic  character  of  the  rocks,  are  strikingly  dissimilar 
in  that  they  are  dominantly  sodic.  Most  of  those  around  the  borders  of  the  Pacific 
Ocean,  those  of  Great  Britain  and  Iceland,  those  of  Hungary  and  the  Grecian 
Archipelago  are  all  much  more  prominently  calcic  and  with  soda  dominating  potash, 
and  in  some  cases  with  highly  quaric  magmas.  But  it  is  needless  to  cite  more 
examples  of  the  dissimilar  ones. 

In  certain  respects  the  Bohemian  Region,  so  ably  described  by  Hibsch,  and 
that  of  the  Eifel  and  Laacher  See,  of  the  rocks  of  which  satisfactory  analyses  are 
singularly  few,  resemble  ours.  Leucitic  rocks  are  present  to  some  extent  and  the 
magmas  are  for  the  most  part  highly  alkalic,  but  here  also  the  soda  dominates  the 
potash,  giving  rise  to  many  highly  nephelinic  and  orthoclase-poor  types  of  rock. 

The  only  sufficiently  known  region  which  is  strikingly  analogous  to  the  Roman 
one  is  that  of  central  Montana,  the  knowledge  of  which  we  owe  to  the  labors  of 
Weed  and  Pirsson,  and  the  general  characters  of  which  have  recently  been  summar- 
ized by  the  latter.* 

While  the  geologic  occurrence  of  the  rocks  of  this  is  very  largely  intrusive,  in 
the  form  of  laccoliths  and  stocks  with  their  attendant  dikes  and  comparatively 

*  L.  V.  Pirsson,  Am.  Jour.  Sci.,  XX,  1905,  p.  35. 

177 


178  THE  ROMAN  COMAGMATIC  REGION. 

few  flows,  the  chemical  and  mineralogical  features  are  very  similar,  though  the 
are  some  marked  differences. 

If  the  magmatic  positions,  which  express  concisely  the  general  chemical  cha 
acters,  of  the  various  analyzed  rocks  of  the  Central  Montana  Region  be  examined 
and  compared  with  that  on  p.n,  the  main  points  are  clearly  seen. 

In  the  first  place,  there  is  a  considerably  greater  variety  of  magmas  than  in  the 
Roman  region,  27  being  represented  in  central  Montana  against  17  known  in  central 
Italy.  But  the  general  range  is  much  the  same.  There  are  rocks  belonging  to 
the  persalane,  dosalane,  and  salfemane  classes,  but  none  in  the  dofemane  (except 
one  on  the  border  of  salfemane)  or  perfemane  classes.  While  there  are  some  rocks 
which  belong  to  the  quaric  orders,  britannare  and  austrare,  in  the  persalane  and 
dosalane  classes,  the  great  majority  are  either  perfelic  or  are  more  or  less  lenic,  and 
it  is  noteworthy  that  in  both  regions  rocks  containing  notable  amounts  of  norma- 
tive lenads  are  rare  in  the  persalane  class,  become  more  frequent  in  the  dosalane, 
and  are  most  so  in  the  salfemane.  Peralkalic  rangs  are  likewise  unusual,  though 
more  common  in  the  Montana  Region  than  in  the  Roman,  and  in  the  former  they 
are  met  with  in  rocks  of  the  dosalane  and  salfemane  classes,  which  is  not  the  case 
in  the  latter.  Alkalicalcic  rangs  are  rare  in  both,  and  in  both  also  the  rangs  most 
commonly  met  with  are  domalkalic.  In  the  subrangs  the  majority  of  those  of  the 
Montana  Region  are  sodipotassic,  but  with  a  very  considerable  representation  of 
dopotassic  ones,  while  we  have  seen  the  reverse  to  be  true  of  the  Roman  Region, 
and  in  addition  a  few  dosodic  subrangs  are  found  in  Montana  sporadically,  which 
are  entirely  absent  in  the  Italian  one.*  But  on  the  whole  potash  can  be  said  to 
dominate  over  soda  in  both,  though  this  is  less  strongly  marked  in  the  Montana 
rocks.  The  resemblance  between  the  two  regions  is  also  shown  among  the  minor 
constituents  by  the  prevalence  of  high  BaO  in  the  rocks  of  both,  a  point  which 
will  be  referred  to  later. 

Similarly,  so  far  as  the  serial  chemical  characters  are  concerned,  we  find  in 
both  a  great  agreement  in  that  potash  increases  relatively  to  soda  as  silica  decreases 
and  as  the  femic  components  increase,!  and  also  that  magnesia  increases  over 
ferrous  oxide  in  the  same  direction.  It  will  also  be  noted  that  in  both  the  general 
geographical  distribution  of  magmas  is  much  the  same.  The  border  or  end  dis- 
tricts are  more  highly  siliceous  and  salic  and  with  more  soda  relatively  to  potash 
than  the  central  districts,  which  are  much  more  distinctly  femic  and  lenic  and  more 
markedly  dopotassic. 

The  average  magma  of  the  Central  Montana  Region  has  not  been  calculated 
by  Pirsson,  but  he  gives  J  an  estimate  of  that  of  the  Highwood  District,  a  comparison 
of  which  with  the  averages  of  the  magma  of  the  Roman  Region  and  of  the  Cam- 
panian  District  is  not  without  interest. 

*  It  is  worthy  of  note  that  dosodic  subrangs  are  very  common  in  the  Crazy  Mountains,  which  Pirsson  does 
not  include  in  his  region.    The  rocks  of  these  evidently  belong  to  a  different  and  more  highly  sodic  magma, 
t  Pirsson,  op.  cit.,  p.  43. 
%  Pirsson,  Bull.  U.  S.  Geol.  Surv.  No.  237,  1905,  p.  193. 


COMPARISON  WITH  OTHER  REGIONS. 

Magmas  of  Roman,  Campanian,  and  Central  Montana  Regions. 


179 


AVERAGE  ROMAN 

M.V.MA. 

AVERAGE  CAM- 
PANIAN MAGMA. 

AVERAGE  HIGH- 
WOOD  MAGMA. 

SiOj  

53.4 

t>4-o 

Co  .  2 

AUO» 

18.3 

18.2 

re  .  7 

Fe.O, 

2.O 

2.6 

37 

FeO     •  

7.4 

7.O 

4      A 

MeO 

7.  I 

2.  7 

52 

CaO    

6.4 

5.6 

7  8 

Na2O    

3.0 

4.  ? 

37 

KaO        

8.3 

7-  $ 

6  i 

TiOj               

0.8 

0.8 

PaOc.. 

0.4 

0.4 

IOO.O 

IOO.O 

97.6 

(II.  6.  2.  2) 

(II.  6.2.  3) 

II.6.2.3) 

In  general  features  the  resemblance  between  the  Italian  and  the  Montana 
magmas  is  close,  though  it  must  be  remembered  that  in  III  we  are  not  dealing  with 
the  magma  of  the  whole  region,  but  with  that  of  a  single  district.  It  is  clear  that 
the  last  is  distinctly  higher  in  the  femic  constituents,  the  two  oxides  of  iron,  mag- 
nesia, and  lime,  and  lower  in  alumina  and  potash,  silica  and  soda  being  about  the 
same  in  both. 

Mineralogically  also  there  are  some  features  in  common.  One  of  the  most 
striking  is  the  occurrence  of  many  leucite-rich  types  in  both  regions,  though  these 
are  much  more  abundant  in  the  Roman  Region.  This  is  undoubtedly  connected 
with  the  more  potassic  character  of  its  magmas,  and  possibly  also  with  the  difference 
in  the  geologic  occurrences  of  the  rocks.  Another  is  the  predominance  in  both 
regions  of  augite  as  the  alferric  mineral.  The  color  and  other  physical  properties 
differ  somewhat,  it  is  true,  but  in  both  cases  this  mineral,  though  titaniferous, 
does  not  show  the  purple  tones  found  elsewhere,  and  the  constant  occurrence  of 
a  diopside-augite  in  two  comagmatic  regions  so  closely  alike  magmatically  is  certainly 
a  very  striking  fact,  and  possibly  one  of  some  significance. 

Again,  while  biotite  is  rather  more  common  in  the  Montana  Region,  possibly 
because  of  the  intrusive  nature  of  most  of  the  Montana  rocks,  it  is  not  an  abundant 
mineral  in  either,  while  the  hornblendes  are  rare  in  both  regions.  Similarly  the 
alkali-feldspar  is  invariably  a  soda-orthoclase,  and  microcline  and  microperthitic 
intergrowths  are  equally  rare  in  both. 

These  comparisons  could  be  carried  further  and  with  greater  detail,  but  the 
above  must  suffice  for  the  present.  It  shows  clearly  that,  while  there  are  certain 
constant  differences,  there  is  a  very  great  degree  of  similarity  between  the  two 
regions,  not  only  in  the  absolute  and  serial  chemical  and  the  mineralogical  charac- 
ters, but  in  the  distribution  in  space  of  the  various  magmas  and  rock  types. 

The  bearing  of  these  and  similar  facts  upon  the  general  questions  of  differen- 
tiation, its  causes,  conditions,  and  processes,  is  as  yet  very  obscure.  As  Pirsson 


i8o  THE  ROMAN  COMAGMATIC  REGION. 

has  pointed  out,  these  are  all  undoubtedly  complex  in  most  cases,  if  not  in  all, 
especially  when  differentiation  over  large  areas  is  involved,  and  the  difficulties  in 
the  way  of  solution  of  the  problems  are  certainly  great.  But  the  discovery  of  such 
similarities  as  have  been  shown  above,  and  the  undoubted  occurrence  of  similar- 
ities of  a  different  sort  obtaining  between  similar  comagmatic  regions  of  widely 
distinct  characters  from  the  two  here  discussed,  indicate  the  truth  of  Pirsson's  con- 
clusion that  "the  distribution  and  occurrence  of  igneous  rocks  are  not  due  to  mere 
chance,"  and  that  it  is  unreasonable  "to  deny  that  they  are  governed  like  other 
things  in  nature  by  definite  laws  and  processes." 

A  much  larger  body  of  data  than  is  yet  available  must  be  collected  before  we 
are  in  a  position  to  ascertain  the  many  facts  of  differentiation  and  to  understand 
its  laws.  This  is  true  even  of  the  smaller  and  less  complex  rock-masses,  such  as 
dikes  and  laccoliths,  where  the  chemical  and  physical  factors  involved  are  com- 
paratively simple.  It  is  a  fortiori  still  more  true  of  such  large  areas  and  masses  as 
those  of  comagmatic  regions.  In  these  we  are  confronted  with  such  difficulties 
in  the  way  of  complexities  of  distribution  and  structure;  complexity,  variety,  and 
mass  of  magmas;  the  possibility  and,  indeed,  the  probability  of  superposed  and 
diverse  processes  and  conditions;  considerations  of  space  and  time;  the  certainty 
of  some  known  physical  factors,  as  viscosity  and  convection  currents,  and  the 
probability  of  some  unknown  ones  entering  into  the  problem;  that  we  must  for  the 
present  be  content  to  observe  and  collect  facts,  leaving  their  collation  and  the  final 
generalizations  from  them  for  a  future  period. 


THE  FORMATION  OF  LEUCITE. 

Although  it  is  now  well  recognized  that  a  magma  of  a  certain  composition 
may  crystallize  as  very  diverse  aggregates,  the  variations  depending  largely  if  not 
entirely  upon  the  conditions  obtaining  during  solidification,  it  is  nevertheless  true 
that  the  chemical  composition  of  the  magma  controls  the  mineral  composition  of 
the  rock  within  limits.  Without  going  into  a  general  discussion  of  this  interesting 
topic,  the  special  case  of  the  relation  between  the  occurrence  of  leucite  in  igneous 
rocks  and  the  composition  of  the  magma  may  be  investigated. 

As  to  the  occurrence  of  this  mineral  the  only  general  "law"  which  has  yet  been 
recognized  is  that  leucite,  as  well  as  nephelite  and  melilite,  are  never  found  in  asso- 
ciation with  quartz,*  that  is,  in  rocks  with  an  excess  of  silica.  It  is  also  generally 
understood  that  it  occurs  most  frequently  in  rocks  which  are  rich  in  potash,  but 
the  limitations  of  this  term  are  very  vague,  and  it  is  known  to  occur  in  rocks  whose 
potash  content  can  not  be  called  high. 

For  the  study  of  this  relation  from  the  side  of  actual  occurrences  the  Roman 
Region  is  especially  favorable.  The  number  and  variety  of  its  leucitic  rocks  are 
great  and  they  have  a  wide  range  in  chemical  composition;  both  leucitic  and  non- 
leucitic  rocks  occur  here  of  similar  as  well  as  of  different  chemical  compositions; 
and  the  disturbing  factor  of  differences  in  physical  conditions  during  solidification 
is  almost  entirely  eliminated,  or  at  least  minimized,  by  the  uniform  occurrence 
of  the  rocks  as  effusive  lava  flows.  We  may,  then,  examine  the  rocks  belonging  to 
magmas  of  the  various  kinds  as  to  the  presence  of  leucite,  drawing  such  conclusions 
as  we  may  from  these  data.  The  full  theoretical  discusssion  of  the  problem  will 
be  published  elsewhere,  probably  in  the  Journal  of  Geology. 

The  Roman  Region. 

Among  the  persalane  rocks  of  the  Roman  Region  leucite  may  be  said  to  be 
non-existent  in  those  which  belong  to  the  sodipotassic  subrang  phlegrose  (I.  5.  i.  3), 
though  it  occurs  very  sporadically  in  the  blocks  of  cumal  phlegrose  at  the  Vico 
Volcano.  The  mineral  begins  to  appear  in  notable  amount  in  the  dopotassic 
vulsinose  rocks  (I.  5.  2.  2),  many  of  which  are  highly  leucitic,  with  the  viterboid 
habit,  though  the  majority  of  the  types  of  this  subrang  are  non-leucitic.  The  same 
is  true  of  the  rarer  sodipotassic  pulaskose  (I.  5.  2.  3)  rocks,  while  in  the  types 
belonging  to  beemerose  (I.  6.  i.  3),  procenose  (I.  6.  2.  3),  and  appianose  (I.  7. 
i.  3)  leucite  is  a  constant  and  important  constituent. 

In  the  dosalanes  it  is,  of  course,  absent  from  the  sorianal  harzose  (II.  4.  3.  3), 
as  there  is  an  excess  of  silica  in  this  magma.  The  ciminose  (II.  5.  2.  2)  rocks 

*  Cf.  F.  Zirkel,  Lehrbuch  der  Petrographie,  Leipzig,  1893,  I,  p.  646. 

181 


1 82  THE  ROMAN  COMAGMATIC  REGION. 

resemble  those  of  vulsinose  in  that  some  of  the  types  are  non-leucitic  and  some 
highly  leucitic,  this  mineral  assuming  the  form  either  of  large  phenocrysts  or  of 
small  individuals  in  the  groundmass.  In  these,  as  well  as  in  the  preceding  cases 
of  persalane  rocks,  it  is  to  be  noted  that  the  modal  leucite  is  formed  at  the  expense 
of  normative  nephelite,  leucite  not  being  found  in  the  norms,  except  in  the  tavolatal 
appianose.  In  the  closely  similar  but  sodipotassic  monzonose  (II.  5.  2.  3)  rocks 
leucite  is  not  found,  except  very  sporadically  in  that  from  L  'Arso,  while  in  the  few 
rocks  which  belong  to  auruncose  (II.  5.  3.  2)  leucite  is  uniformly  present  in  abun- 
dance. In  the  only  rocks  of  this  region  so  far  known  which  belong  to  shoshonose 
(II.  5.  3.  3)  leucite  is  wanting. 

With  the  rocks  belonging  to  vicose  (II.  6.  2.  2)  we  meet  leucite  both  in  the 
norm  and  in  the  mode,  and  from  here  on  to  the  end  of  the  series  leucite  is  a  constant 
and  important  constituent,  no  non-leucitic  types  of  vicose  (II.  6.  2.  2),  braccianose 
(II.  7.  2.  2),  vesuvose  (II.  8.  2.  2),  or  of  the  salfemane  subrangs  jugose  (III.  7. 
2.  2),  fiasconose  (III.  7.  3.  2),  or  albanose  (III.  8.  2.  2)  being  known  in  the  Roman 
Region. 

From  these  facts  we  may  draw  the  following  conclusions  as  to  the  formation 
of  leucite  in  effusive  rocks,  in  the  Roman  Region  at  least :  It  will  not  be  formed 
in  rocks  belonging  to  quaric  orders,  or  those  with  an  excess  of  silica  (norm- 
ative quartz).  It  is  not  liable  to  be  formed  in  sodipotassic  magmas,  especially 
when  of  the  dosalane  class  and  in  persalanes  when  the  norm  is  perfelic.  But  this 
prohibitive  influence  of  the  abundance  of  soda  is  counteracted  by  paucity  in  silica, 
as  in  beemerose,  procenose,  and  appianose  rocks,  whose  norms  all  show  notable 
amounts  of  lenads. 

In  magmas  which  belong  to  perfelic  orders  and  to  dopotassic  subrangs,  whether 
in  persalane  or  dosalane,  as  vulsinose  (I.  5.  2.  2)  and  ciminose  (II.  5.  2.  2),  leucite 
may  or  may  not  be  formed,  and  these  magmas  seem  to  be  in  a  somewhat  critical 
condition  as  regards  the  occurrence  of  this  mineral.  The  occurrences  are  all 
extrusive  lava  flows  and  no  constant  differences  in  the  conditions  obtaining  during 
solidification  can  be  made  out  for  the  leucitic  and  the  non-leucitic  flows,  nor  can 
any  definite  order  of  succession  be  established.  From  the  chemical  side  the  analy- 
ses at  hand  would  lead  us  to  suppose  that  leucite  will  not  be  formed  if  the  silica 
is  above  56,  assuming  that  8  or  9  per  cent  of  potash  is  present,  though  it  can  form 
with  this  amount  of  silica  if  the  potash  percentage  runs  above  10  (with  about  3  or 
4  per  cent  of  soda).  Stated  in  other  terms,  if  no  nephelite,  or  only  i  or  2  per  cent, 
is  found  in  the  norm  no  leucite  or  only  sporadic  crystals  will  occur  in  the  rock. 

In  the  subrangs  of  dosalane  which  show  notable  amounts  of  nephelite  in  the 
norm  and  in  the  similar  ones  of  salfemane,  all  of  these  being  dopotassic,  leucite 
invariably  occurs  in  the  mode. 

Although  the  number  of  chemical  factors  possibly  involved  is  large,  and  the 
possibilities  of  mutual  readjustments  between  normative  mineral  molecules,  as 
between  hypersthene  and  olivine,  are  therefore  complex,  it  would  seem  that  the 


THE  FORMATION  OF  LEUCITE. 


183 


relations  between  silica  and  potash  on  the  one  hand,  and  between  potash  and  soda 
on  the  other,  are  largely  the  determining  factors  in  the  rocks  of  the  region,  the  con- 
ditions of  solidification  being  similar.  The  data  on  which  to  study  this  question 
are  given  in  the  annexed  table,  in  which  the  amounts  of  silica  and  potash  are  given 
both  in  percentages  and  as  molecular  ratios,  together  with  the  ratios  (molecular) 

=—4  and  '  _  .  The  rock  numbers  are  the  same  as  in  the  table  of  analyses 
K2O  Na2O 

on  p.  146,  and  the  leucitic  rocks  are  indicated  by  an  asterisk  (*),  those  in  which  the 
mineral  is  merely  sporadic  and  accessory,  as  the  cumal  phlegrose  and  arsal  mon- 
zonose,  being  considered  non-leucitic. 

Relations  of  Silica,  Potash,  and  Soda  in  the  Roman  Rocks. 


c;r« 

Ko 

SiO. 

K,O 

c:in 

Kr> 

SiO, 

K,O 

K,O 

Na.O 

K.O 

Na,O 

I  

6l.88 

6.  72 

U.  S2 

0.64 

23  .  . 

c6.  7? 

5  .02 

TC.  .02 

0.83 

2  

en.  70 

7.  IO 

13.12 

0.60 

24  

C.4..  72 

6.87 

12.40 

I.  3o 

60.  33 

7.  3O 

12  QO 

O.  73 

2S*  .  . 

C2.  37 

7.47 

IO  OI 

I.  7O 

4  .  . 

CQ.  24 

0.  14 

IO.  17 

1  .  23 

26*  

Co.  86 

7.  IS 

II  .OI 

2.  2O 

c  .  . 

61  .  62 

7.6O 

12.68 

0.87 

27*  .  . 

CT  .  21 

6.60 

12  .  2O 

I.  7S 

6  

S8.o8 

8.87 

2.  O7 

1  .07 

28  

cc  .60 

4.41 

IO.  74 

I  .OO 

7  

C7.e8 

8.68 

IO.  32 

1.86 

20*  .  . 

54-83 

10.40 

8.23 

2.  36 

8  ..  •  ... 

c.7.  co 

8.  30 

10.78 

I.  74 

3O*  .  . 

C.I  .  20 

IO.63 

7.  SS 

3-  32 

9*  

c6.  IQ 

IO.4-7 

8.36 

2.43 

31*  .. 

So.  68 

0-38 

8.45 

2.38 

10*  

cc.  17 

o.c8 

9  .02 

2.  76 

32*  .  . 

co.  36 

Q.  5Q 

8.30 

3.  13 

ii  

C7.6o 

8.71 

IO.O2 

1.63 

33*  .. 

CO.  24 

7.4S 

10.48 

2.OO 

12*  

CC.O7 

8.65 

0.87 

1.4.7 

24*  

47.6? 

7.47 

9.QO 

1.78 

13*  .- 

ee.87 

10.40 

8.31 

I.4S 

35*  .. 

47.8o 

8.23 

Q-l8 

2.08 

14*  -. 

CO.  2S 

II  .  32 

6.08 

1  .43 

36*  .. 

47.  20 

7.63 

O.  72 

2.  2S 

je  .  . 

SO.  41 

c  .  20 

17.68 

1  .  33 

37*  .. 

48.  10 

7.OO 

o.  cc 

1  .01 

16  

S7.  32 

O.  IS 

0.84 

1.87 

38*  .. 

47.  71 

7.64 

Q.8l 

1.  80 

17  •  • 

ce.46 

6.63 

I3.OI 

2.4? 

30*  .  . 

47-  30 

6.Q3 

10.82 

3.O4 

18  

S7-  31 

6.38 

10.04 

3-OO 

4O*  .  . 

44.8o 

3-63 

IO.  O2 

2.20 

10*  .  . 

re.  8? 

8.77 

Q.OO 

I.  71 

41*  .. 

46.  24 

6.37 

II.  11 

2.  S2 

20*  .... 

cc  .  21 

8.4.C. 

IO.  22 

1.  80 

42*  .. 

46.  27 

8.s8 

8.47 

2.  O7 

21*  

S2.  14 

7.24 

II  .  20 

2.48 

43*  

47.  OS 

7-  S2 

O.SQ 

3.O7 

22  

55-22 

7.58 

II.  36 

1  .  4C 

4-4*  .  . 

4S  .00 

8.07 

8.00 

2.74 

Regarded  thus  in  tabular  form  these  figures  seem  to  point  to  no  very  definite 

C  */*"\ 

law.      While  the  non-leucitic  rocks  show  generally  high   _    *  ratios,  we  find 

jVjjL) 

almost  equally  high  figures  for  this    (19.74  and  19.02)  in  the  non-leucitic  and 

K  O 
the  leucitic  types.     Similarly  the  other  ratio,      2      ,  does  not  show  any  very  marked 

JN  clj  \J 

regularity.  But  if  the  figures  are  plotted  as  in  the  subjoined  diagram,  the  abscissae 
representing  the  percentage  of  silica  and  the  ordinates  the  ratios  in  question,  some 
interesting  relations  are  made  manifest.  The  dots  represent  non-leucitic  rocks 
and  the  dots  in  circles  leucitic  ones. 


1 84 


THE  ROMAN  COMAGMATIC  REGION. 


In  the  first  place,  as  regards  the  amounts  of  silica  alone  some  very  decided 
breaks  in  the  continuity  are  visible,  as  has  been  mentioned  on  a  previous  page. 
These  are  especially  marked  and  wide  between  48  and  50  per  cent,  and  between 
52.5  and  54 . 6.  There  are  narrower  ones  between  56 . 2  and  57.3  (with  one  analysis 
in  the  gap),  and  between  54.7  and  59.2,  though  probably  the  series  of  silica  per- 
centages between  54.7  and  61 .9  may  be  regarded  as  continuous.  As  the  number 
of  analyses  is  rather  large,  it  would  be  reasonable  to  suppose  that  the  two  large 
breaks  are  real  and  that  they  point  to  some  definite  differentiation  of  the  general 
magma  into  at  least  three  submagmas.  It  would  be  of  very  great  interest  to  discover 
if  similar  clusterings,  or  indications  of  the  presence  of  submagmas,  are  to  be  found 
in  other  regions.  There  are  some  indications  that  such  is  the  case  elsewhere,  but 


46  48  SO  52  64  £6  59  SO 

•  «  Non  Isucitic  rocks         •  =  Leucitic  rocks 
FIG.   2. 

this  is  a  matter  the  investigation  of  which  must  be  deferred  for  the  present,  as  it 
too  general  in  scope  for  the  purposes  of  this  paper. 

Returning  to  the  immediate  question  at  issue,  it  will  be  seen  that  the  rocks 
below  52.5  of  silica  are  uniformly  leucitic,  while  those  above  56.3  are  as  uniformly 
non-leucitic,  and  those  with  silica  between  54.6  and  56.3  are  of  both  leucitic  and 
non-leucitic  types. 

Confining  our  attention  to  the  rocks  with  silica  above  54 . 6,  it  is  clear  that  the 
silica-potash  ratios  of  the  non-leucitic  rocks  are  uniformly  above  those  of  the  leucitic 
ones  in  the  group  between  54.6  and  56.3,  which  may  be  called  the  critical  group, 
and  which  embraces  most  of  the  types  of  vulsinose  and  ciminose.  To  the  right 
of  this  the  silica-potash  ratios  tend  to  decrease  and  even  reach  the  level  of  or  fall 
below  the  ratios  of  the  leucitic  rocks  of  the  critical  group.  The  division  line  between 
them  may  therefore  be  regarded  as  a  curve  which  inclines  downward  gently  to  the 
right,  that  is,  as  silica  increases,  slightly  above,  but  parallel  to,  the  line  A-B  of 


THE  FORMATION  OF  LEUCITE.  185 

the  diagram,  which  is  drawn  from  data  furnished  by  a  theoretical  discussion  of 
the  subject  and  which  will  be  referred  to  later. 

The  relations  of  the  potash-soda  ratios  are  not  so  clear,  especially  in  the  critical 
group,  where  they  are  quite  involved,  those  of  leucitic  and  non-leucitic  rocks  being 
both  above  and  below  each  other.  But  the  tendency  to  high  figures  as  silica 
decreases  and  as  leucite  becomes  constant,  and  to  low  figures  as  silica  increases 
and  as  leucite  vanishes,  is  very  marked,  and  seems  to  admit  of  no  question. 

In  regard  to  the  so-called  critical  magmas,  the  possibly  significant  fact  may 
be  pointed  out  that  its  limits  of  variation  in  the  percentage  of  silica  are  practically 
conterminous  with  those  shown  by  the  analyses  of  natural  leucites,  which  are  54  .  4 
and  57.2  in  the  better  analyses.*  The  best  analysis  of  the  leucite  of  Vesuvius,! 

with  SiOa  =  55.40  and  ^-^=4.44,  falls  just  beneath  the  center  of  the  critical 

S'O 

group,  while  the  theoretical  leucite  molecule,  with  SiO2  =  55-o5  and  =^—-^=4.00, 

J\.2O 

lies  a  trifle  to  the  left  of  this.  The  positions  of  the  two  are  indicated  respectively 
in  the  diagram  by  a  small  cross  (+)  and  by  a  star  (*).  The  position  of  the  average 
magma  of  the  region  is  indicated  by  the  sign  X  • 

Conclusions. 

From  the  facts  set  forth  in  the  preceding  pages  the  following  conclusions  as  to 
the  relation  of  the  magma  tic  composition  to  the  occurrence  of  leucite  in  effusive, 
holocrystalline,  igneous  rocks  can  be  drawn.  They  seem  to  hold  good  for  the 
Roman  Region,  but  whether  they  are  of  more  general  application  or  not  must  be 
determined  by  future  investigation.  The  more  special  conclusions  are  first  given, 
followed  by  those  of  a  more  general  character. 

1.  Leucite  forms  invariably  in  magmas  with  SiO,  less  than  52.50  per  cent, 

the  ratio         '  being  lower  than  13  .o,  though  it  may  be  higher  than  this.    (Analy- 
Jv2O 

tical  data  are  wanting  in  regard  to  this  last  point.) 

2.  Leucite  is  invariably  absent,  or  present  only  very  sporadically,  when  the 

silica  is  above  56  .  5  and  with  the  ratio         '  at  least  10  or  higher,  the  upper  limit 

J\.2O 

being  infinity.     (Analytical  data  are  wanting  for  ratios  considerably  lower  than  10.) 

3.  In  magmas  with  silica  varying  from  54.7  to  56.3,  or  those  in  which  the 

C  "  f*\ 

silica  percentages  are  approximatelv  those  of  the  natural  leucites,  ^-~  varying 

J\.2O 

from  20  to  less  than  10,  leucite  may  or  may  not  occur,  and  these  magmas  may  be 
regarded  as  critical  ones. 


4.  In  the  critical  magmas  the  ratio  ^p—A  is  uniformly  higher,  for  any  given 

JVjVj 

*  Cf.  Dana,  Mineralogy,  1892,  p.  342,  and  Hintze,  Mineralogie,  II,  1895,  p.  1311. 
t  G.  Steiger,  analyst,     Bull.  U.  S.  Geol.  Surv.  No.  220,  1903,  p.  30. 


i86 


THE  ROMAN  COMAGMATIC  REGION. 


silica  percentage,  in  the  non-leucitic  than  in  the  leucitic  rocks,  the  division  line  as 
expressed  on  the  diagram  falling  slowly  as  silica  increases  and  rising  as  silica 
decreases.  It  is  probable  that  this  division  line  extends  an  indefinite  distance  in 
both  directions  beyond  the  limits  of  the  critical  group,  but  analytical  data  are 
wanting  to  determine  this. 

K  O 

5.  Leucite  is  most  apt  to  occur  in  the  mode  when  the  ratio      '  x  is  greater 

Na2O 

than  unity  (perpotassic  and  dopotassic  subrangs),  the  tendency  to  the  formation 
of  leucite  being  proportional  to  the  ratio;  leucite  is  not  apt  to  be  formed  if  the  ratio 
approximates  to  unity  (sodipotassic  subrangs);  and  leucite  does  not  form  when 
the  ratio  is  less  than  unity  (dosodic  and  persodic  subrangs). 

6.  The  prohibitive  tendency  of  high  soda  may  be  overcome  by  a  low  ratio  of 
silica  to  potash. 


v  e 

©•    © 


z% 


•  -  Non  leucitic  rocks        e  =  Leucitic  rocks 
FIG.  3. 


The  more  general  conclusions  which  may  be  drawn  are: 

7.  The  formation  of  leucite,  or  its  modal  occurrence,  is  directly  dependent 
upon  the  chemical  composition  of  the  magma. 

8.  The  chief  chemical  factors  involved  are  the  percentage  of  silica,  the  ratio 
of  silica  to  potash,  and  the  ratio  of  potash  to  soda.     The  relations  of  the  other 
constituents  are  apparently  of  subordinate  importance,  if  not  quite  negligible  in 
some  cases. 

9.  The  ratio  of  potash  to  soda  is  of  subordinate  importance  to  that  of  silica 
to  potash. 

10.  Leucite  does  not  occur  in  rocks  with  an  excess  of  silica,  or  more  than 
sufficient  to  saturate  the  bases  completely  and  form  the  most  highly  silicated  mineral 
molecules  possible,  that  is,  in  magmas  where  quartz  would  occur  in  holocrystal- 
line  types. 


THE  FORMATION  OF  LEUCITE.  187 

11.  The  presence  in  the  norm  of  such  mineral  molecules  as  nephelite  or  olivine, 
which  are  capable  of  being  more  highly  silicated,  is  a  necessary  condition  for  the 
formation  of  leucite  if  there  is  no  normative  leucite.      Of  these  nephelite  seems  to 
be  the  most  important,  modal  leucite  being  usually  formed  by  a  readjustment  of 
silica  involving  the  formation  of  modal  albite  from  normative  nephelite. 

12.  When  leucite  is  found  in  the  norm  of  a  rock  it  is  also  present  in  the  mode. 

13.  It  is  probable  that  the  formation  of  leucite  anticipates  and  determines  the 
distribution  of  other  mineral  molecules,  as  those  of  Na2O  in  albite  (as  modal  plagio- 
clase  rather  than  nephelite,  or  as  (Fe,Mg)O  in  hypersthene  (as  modal  augite) 
rather  than  olivine. 

Returning  for  a  moment  to  the  diagram,  and  anticipating  part  of  a  theoretical 
discussion  which  will  be  published  elsewhere,  it  may  be  pointed  out  that  the  line 

C  *  f\ 

A-B  represents  the  limiting  value  of  the  ratio  „  ^  for  persalic  magmas,  in  which 

Jv3U 

leucite  would  be  present  in  the  norm  (and  hence  also  in  the  mode  according  to  12 
above),  on  the  assumption  that  the  affinity  of  potash  for  silica  is  greater  than  that 
of  soda.  For  a  given  silica  percentage,  if  the  ratio  in  question  falls  below  this  line 
the  rock  will  be  leucitic.  If  the  ratio  is  above  this  line  for  any  given  silica  per- 
centage the  rock  may  be  leucitic  actually,  but  is  not  necessarily  so,  the  tendency 
to  the  formation  of  leucite  in  the  mode  decreasing  rapidly  as  the  silica-potash  ratio 
rises  above  the  line. 

It  appears  from  this  that  the  so-called  "critical  group"  is,  in  reality,  one  of 
rocks  whose  silica-potash  ratios  fall  very  close  to  the  limiting  curve  for  the  forma- 
tion of  leucite,  and  that  the  apparently  sensitive  or  nearly  balanced  chemical  con- 
dition of  their  magmas,  where  very  slight  changes  in  the  physical  or  physico-chem- 
ical conditions  would  either  bring  about  or  prevent  the  formation  of  leucite,  is 
presumably  due  to  this  fact. 

The  line  C-D  indicates  the  limiting  values  in  persalic  magmas  of  the  possible 
formation  of  leucite  due  to  deficient  silica.  Below  it  for  any  given  silica  percentage 
there  is  an  insufficient  amount  of  silica  for  even  leucite  and  nephelite,  and  hence 
such  magmas  would  seem  to  be  incapable  of  existence,  unless  such  a  mineral  as 
kaliophilite  (K2O.Al2O3.2SiO2),  were  present.  It  is  interesting  to  note  that  the 
only  two  analyses  which  fall  below  it  here  are  Nos.  42  and  44  on  page  146,  the  rocks 
corresponding  to  which  contain  notable  amounts  of  akermanite  in  the  norm,  and 
of  modal  melilite  in  one  case  at  least. 


THE  DISTRIBUTION  OF  BARIUM. 

In  the  comparison  of  the  Roman  and  Central  Montana  Regions  it  was  noted 
that  the  rocks  of  both  regions  are  characterized  by  the  presence  of  large  amounts 
of  barium,  large,  that  is,  for  this  constituent,  the  amount  of  which  in  igneous  rocks 
is  very  seldom  over  i  per  cent  and  usually  less  than  one-tenth  of  this.  It  will  be 
appropriate,  therefore,  in  this  connection  to  discuss  briefly  the  distribution  of  barium 
among  igneous  rocks. 

Hillebrand*  and  Vogtf  have  called  attention  to  its  widespread  occurrence,  and 
the  former  has  pointed  outj  the  local  variation  of  the  igneous  rocks  of  the  United 
States  in  regard  to  this  constituent,  in  that  those  "of  the  Rocky  Mountain  region 
show  far  higher  average  percentages  than  the  rocks  from  the  eastern  and  more 
western  portions."  It  will  be  of  interest  to  examine  its  distribution  from  another 
point  of  view,  not  so  much  that  of  the  locality  as  in  relation  to  the  other  chemical 
characters  of  the  magmas  or  comagmatic  regions. 

Our  data  for  such  a  discussion  are  unfortunately  somewhat  scanty.  In  the 
igneous  rocks  of  the  United  States  BaO  has  been  determined  very  frequently  by 
the  chemists  of  the  Geological  Survey,  especially  in  the  more  recent  analyses,  where 
it  is  invariably  looked  for.  It  is  also  regularly  determined  by  the  chemists  of  the 
Geological  Survey  of  New  South  Wales,  and  has  been  determined  frequently  in 
the  rocks  of  British  Guiana  by  Professor  J.  B.  Harrison.  For  the  igneous  rocks 
of  Europe,  as  well  as  of  Asia  and  Africa,  determinations  of  BaO  are  almost  non- 
existent, those  presented  in  this  paper  being  about  the  only  ones  to  be  found.  In- 
deed BaO,  as  well  as  most  of  the  other  minor  constituents,  have  been  usually  neg- 
lected in  rock  analysis.  Outside  of  the  main  constituents,  TiOa,  P2OS  and  MnO 
are  the  only  minor  ones  to  which  attention  is  commonly  paid. 

The  average  percentage  of  barium  in  the  earth's  crust  was  first  estimated  by 
Clarke§  in  1891,  the  figure,  0.03  per  cent,  being  based  only  on  analyses  of  rocks  of 
the  United  States.  This  was  later  and  successively  ||  raised  to  0.04,  0.05,  and  0.089, 
these  figures  applying  only  to  the  igneous  rocks  of  the  United  States.  The  reason 
for  the  great  difference  between  the  earlier  estimates  and  the  latest  is  the  difference 
in  method  of  computing  the  means.lf  In  the  earlier  estimates  all  the  analyses  were 
averaged  as  if  each  was  complete,  while  in  the  last  the  determinations  for  each  con- 
stituent were  averaged  separately.  As  BaO  was  not  determined  in  many  of  the 
analyses,  this  would  lead  to  low  figures  by  the  former  method. 

*  W.  F.  HiUebrand,  Jour.  Am.  Chem.  Soc.,  XVI,  1894,  p.  81. 
t  J.  H.  L.  Vogt,  Zeits.  prakt.  Geol.,  1898,  p.  231. 
J  W.  F.  HiUebrand,  Bull.  U.  S.  Geol.  Surv.  No.  148,  1897,  p.  19. 
§  F.  W.  Clarke,  Bull.  U.  S.  Geol.  Surv.  No.  78,  1891,  p.  30- 

||  Clarke,  Bulls.  U.  S.  Geol.  Surv.  No.  148,  1897.  p.  13;    No.  168,  1900,  p.  15;  No.  228,  1904,  p.ip. 
II  Clarke.  Bull.  U.  S.  Geol.  Surv.  No.  228,  1004,  p.  16. 
188 


THE  DISTRIBUTION  OF  BARIUM.  189 

Assuming  that  the  value  of  o.n  for  BaO  in  the  igneous  rocks  of  the  United 
States  is  correct,  it  must  be  noted  that  the  data  include  many  analyses  of  rocks  from 
regions  where  this  constituent  is  prominent,  and  to  judge  from  the  meager  indica- 
tions at  hand  for  other  parts  of  the  globe,  it  would  seem  that  the  average  for  the 
earth  as  a  whole  is  considerably  lower  than  this.  Exactly  how  much  less  it  is  im- 
possible to  say,  but  it  is  probable  that  the  average  percentage  of  barium  in  the  igneous 
rocks  of  the  globe  may  be  put  at  about  0.05,  equivalent  to  about  0.06  of  BaO.  While 
this  figure  is  based  on  admittedly  insufficient  data,  it  is  probably  not  far  from  truth, 
and  may  be  accepted  provisionally  for  the  present.  For  Europe,  at  least,  it  is 
undoubtedly  less  than  for  the  United  States. 

Examining  analyses  in  which  BaO  has  been  determined,  those  of  the  United 
States,  New  South  Wales,  British  Guiana,  and  the  Roman  Region,  it  is  found 
that  this  constituent  tends  to  increase  as  silica  decreases,  though  this  is  not  marked, 
and  seems  to  be  subordinate  to  other  relations.  These  are  that  rocks  high  in  BaO 
are  likewise  apt  to  be  high  in  K2O,  especially  if  considerable  CaO  is  also  present. 
On  the  other  hand,  rocks  high  in  Na2O,  and  those  which  are  docalcic  or  percalcic 
(that  is  with  very  subordinate  amounts  of  the  alkalis)  show  very  small  amounts  of 
BaO  as  a  rule. 

All  the  specific  data  can  not,  of  course,  be  cited  here,  but  the  following  illustra- 
tions may  be  given.  In  the  rocks  of  the  Leucite  Hills  in  Wyoming,  which  are  the 
richest  in  potash  as  well  as  in  barium  so  far  known,  with  an  average  percentage  of 
10.53  for  K2O,  BaO  is  0.80,  in  the  Central  Montana  Region  the  average  for  K2O  is 
4.75  and  that  of  BaO  is  0.33,  while  in  the  Roman  Region  the  figures  are  respectively 
7.93  and  0.16.  The  average  percentage  of  K2O  in  central  Montana  may  not  seem 
very  high,  but  it  is  very  considerably  above  that  for  the  igneous  rocks  of  the  United 
States,  2.93,*  or  for  the  earth,  3.16-!  That  for  the  Roman  Region  is  much  above 
this,  and  relatively  high  potash  is  also  shown  by  the  frequent  occurrence  of  leucite 
rocks  in  both  regions.  While  the  figure  for  BaO  in  the  Roman  Region  may  seem 
low,  it  is  above  the  average  one  as  stated  above,  and  certainly  above  that  for  Euro- 
pean rocks  so  far  as  the  data  at  hand  allow  us  to  judge. 

Examples  of  the  converse  of  the  above  among  the  rocks  of  the  United  States 
are  so  numerous  that  it  is  needless  to  cite  instances.  An  examination  of  the  analy- 
ses recorded  in  the  bulletins  mentioned  above  will  disclose  the  frequency  of  figures 
for  BaO  varying  from  o.io  to  none  in  the  rocks  of  comagmatic  regions  where  leu- 
citic  rocks  do  not  occur,  and  where  Na2O  is  the  dominant  alkali  or  where  salic  lime 
dominates  over  the  alkalis.  Similarly  in  British  Guiana  the  analyses  of  Professor 
Harrison  show  an  average  percentage  for  K2O  of  2.47,  while  that  of  BaO  is  about 
0.02,  and  this  latter  constituent  is  reported  as  absent  in  half  the  analyses. 

The  igneous  rocks  of  New  South  Wales   furnish    somewhat   different   results. 
Here  46  excellent  analyses  published  between  1902  and  1905  in  the  records  of  the 

*  Clarke,  Bull.  U.  S.  Geol.  Surv.  No.  228,  1904,  p.  19. 

t  Washington,  Prof.  Paper  U.  S.  Geol.  Surv.  No.  14,  1003,  p.  107. 


1 90  THE  ROMAN  COMAGMATIC  REGION. 

Geological  Survey  show  an  average  for  K2O  of  2.61,  that  for  BaO  being  0.094. 
This  figure  for  K2O  is  distinctly  below  the  average  for  igneous  rocks  as  a  whole, 
while  that  for  BaO  is  somewhat  above  the  average  already  given.  Also  the  rocks 
of  the  country,  so  far  as  known,  are  relatively  high  in  soda,  dosodic  and  sodipotassic 
subrangs  predominating,  and  nephelite  being  quite  common.  But  it  is  notable  that 
the  highest  figures  for  K2O,  0.32  and  0.30,  are  reported  for  the  "leucite-basalts"  of 
Byerock  and  El  Capitan,  and  that  in  the  "oli vine-basalts"  and  " monchiquites "  the 
percentages  of  BaO  are  expressed  in  a  few  hundredths  of  i  per  cent. 

In  greater  detail  it  will  be  found  that  for  any  region  the  variation  of  BaO  corre- 
sponds in  a  general  way  with  that  of  K2O,  though  there  are  exceptions  to  this  in 
all.  From  the  facts  above  we  may  infer,  at  least  provisionally  and  with  due 
recognition  of  the  insufficiency  of  our  data,  that  there  is  some  correspondence 
between  the  K^O  and  the  BaO  in  igneous  rocks,  and  that  the  two  go  hand  in 
hand  more  or  less,  the  detailed  relations  varying  with  the  special  characters  of  the 
region. 

But  while  such  a  general  correspondence  is  thus  indicated,  it  can  not  be  denied 
that  there  are  exceptions  and  that  there  does  not  seem  to  be  any  strict  proportionality 
between  the  amounts  of  the  two  constituents,  at  any  rate  when  different  regions 
are  considered.  This  is  illustrated  by  the  figures  for  the  Roman  and  the  Central 
Montana  Regions,  and  among  the  analyses  of  the  rocks  of  the  United  States,  New 
South  Wales,  and  the  Roman  Region  cases  can  be  found  with  quite  high  figures  for 
BaO,  connected  with  relatively  low  ones  for  K2O. 

It  would  seem,  therefore,  that  the  distribution  of  BaO,  while  almost  certainly 
connected  in  some  way  with  that  of  K2O,  is  dependent  also  upon  more  general 
magmatic  or  regional  characters.  It  is  uncertain  as  yet  whether  the  correspondence 
between  the  two,  assuming  this  to  exist,  is  due  to  some  relation  between  potas- 
sium and  barium  as  elements,  to  the  similarity  of  their  behavior  in  regard  to  the 
physico-chemical  processes  involved  in  the  formation  or  differentiation  of  igneous 
rocks,  or  finally  to  a  mere  coincidence,  the  correspondence  being  brought  about  by 
chance  segregations  in  the  earlier  stages  of  the  formation  of  igneous  magmas  in  the 
body  of  the  earth. 

The  present  case  seems  to  be  analogous  to  the  predilection  of  ZrO2  for  sodic 
rocks  and  that  of  Cr2O3  for  those  high  in  magnesia.  Exceptions  to  both  of  these 
are  known,  in  the  sense  that  highly  sodic  rocks  are  found  with  little  or  no  ZrO2  and 
highly  magnesic  ones  in  which  Cr2O3  is  absent.  But  the  general  correspondence 
between  these  pairs  of  constituents,  as  well  as  between  others  which  might  be  men- 
tioned, is  now  generally  recognized  in  petrology. 

In  view  of  the  common  and  widespread  occurrence  of  such  correspondences, 
the  exceptions  should  not  be  held  to  invalidate  the  final  conclusions  as  to  the  distri- 
butions. They  indicate  rather  that  the  relations  are  more  complex  than  is  now 
apparent,  and  that  the  distributions  of  the  rarer  elements,  as  well  as  of  the  more 
common  ones,  among  igneous  magmas  or  rocks  are  the  resultants  of  many  factors. 


THE  DISTRIBUTION  OF  BARIUM.  191 

Whatever  be  the  true  facts  it  is  clear  that  such  problems  are  of  considerable 
importance,  not  only  to  petrology  but  to  other  sciences  as  well,  and  that  for  their 
solution  complete  chemical  analyses  of  igneous  rocks  and  the  determination  of 
constituents  which  are  usually  disregarded  on  account  of  their  rarity  are  absolutely 
essential. 

As  to  the  minerals  into  which  the  BaO  enters  in  igneous  rocks,  the  data  are  as 
yet  unsatisfactory,  since  many  mineral  analyses  have  suffered,  like  those  of  rocks, 
from  neglect  to  determine  the  rarer  constituents.  As  shown  by  the  analyses  in 
Hintze's  Mineralogie,  by  those  in  Bulletin  No.  220  of  the  U.  S.  Geological  Survey 
(1903),  and  as  pointed  out  by  Vogt  in  the  paper  cited  above,  BaO  has  been  found 
in  many  feldspars,  though  it  has  been  looked  for  only  exceptionally.  It  would  seem 
to  be  most  common  in  orthoclase,  soda-orthoclase,  and  microcline,  presumably  as 
the  hyalophane  molecule,  and  has  also  been  noted  in  a  few  soda-lime  feldspars, 
where  it  doubtless  exists  in  the  molecule  of  celsian.  Its  presence  has  been  detected 
in  both  muscovite  and  biotite,  sometimes  in  very  considerable  amount,  up  to  as  high 
as  6.84  of  BaO,  while  it  seems  to  be  absent  from,  or  present  only  as  traces  in,  the 
pyroxenes  and  amphiboles.  It  will  be  noted  that  these  facts  of  distribution  among 
minerals,  showing  that  BaO  is  most  prominent  in  the  potassic  minerals  and  least 
so  in  those  poor  in  potash,  are  in  line  with  the  observations  as  to  the  distribution 
of  BaO  among  igneous  rocks. 

There  does  not  seem  to  be  any  record  of  the  presence  of  BaO  in  leucite,  at 
least  none  has  been  found  in  a  search  of  the  literature.  A  large  leucite  from  the 
viterbal  vicose  of  Garofali  in  the  Auruncan  District  was  examined,  and  furnished 
0.09  per  cent  of  BaO,  while  the  similar  rock  from  Monte  San  Antonio  gave  0.33  per 
cent.  It  is  expected  to  examine  other  leucites  in  this  regard,  but  it  would  seem 
from  this  result  that  the  BaO  in  the  Italian  rocks  exists,  for  the  most  part,  in  the 
feldspars,  and  only  subordinately  in  the  leucite. 


BIBLIOGRAPHY. 

The  following  list  does  not  aim  at  completeness,  but  simply  presents  the  books 
and  papers,  especially  those  of  more  modern  dates,  which  have  been  found  most 
useful  in  the  present  study.  It  includes  chiefly  those  in  which  the  several  districts 
and  their  igneous  rocks  have  been  described  petrographically,  but  only  very  few 
which  deal  chiefly  or  solely  with  the  structural  and  stratigraphical  geology.  The 
districts  vary  widely  from  a  bibliographical  point  of  view.  In  some  cases,  notably 
the  Campanian  and  Latian  Districts,  the  literature  is  extensive  and  covers  several 
centuries,  while  for  others  the  literature  is  scanty  and  the  number  of  titles  few. 

1.  ABICH,  H.     Geologische  Beobachtungen  Uber  dievulkanischen  Erscheinungen  und  Bildungen 

in  Unter-  und  Mittel-Italien.     Braunschweig,  1841,  I,  Lief,  i,  pp.  1-134. 

2.  BRANCO,  W.     "  Die  Vulkane  des  Herniker-Landes  bei  Frosinone  in  Mittel-Italien. "     Neu. 

Jahrb.,  1877,  pp.  561-589.     (Map.) 

3.  BRANCO,  W.  "I  Vulcani  degli  Ernici  nella  Valle  del  Sacco."     Atti  Ace.  Line.  (3),  I,  1877, 

pp.  801-817. 

4.  BTJCCA,  L.     "  Contribuzione  allo  studio  petrografico  dell'  Agro  Sabatino  e  Cerite."     Boll. 

Com.  Geol.  Ital.,  1886,  pp.  211-223. 

5.  BTJCCA,  L.  "II  Monte  di  Roccamonfina. "     Boll.  Com.  Geol.  Ital.,  1886,  pp.  245-265. 

6.  BTJCCA,  L.     "  Contribuzione  allo  studio  petrografico  dei  Vulcani  Viterbesi. "     Boll.  Com. 

Geol.  Ital.,  1888,  pp.  57-63. 

7.  BTJCCA,  L.     "Studio  petrografico  sulle  trachite  leucitiche  del  Lago  di  Bolsena. "    Riv.  Min. 

Crist,  XII,  1893,  pp.  18-30. 

8.  DEECKE,  W.    "Fossa  Lupara,  ein  Krater  in  den  phlegraischen  Feldern  bei  Neapel."     Zeits. 

d.  deutsch.  geol.  Ges.,  XL,  1888,  pp.  166-181. 

9.  DEECKE,  W.     "Bemerkungen  zur  Entstehungsgeschichte  und  Gesteinskunde  der  Monti 

Cimini."     Neu.  Jahrb.,  B.  B.  VI,  1889,  pp.  205-240. 

10.  DEECKE,  W.     Geologischer  Fiihrer  durch  Campanien.     Berlin,  1901,  pp.  235. 

11.  DELL*  ERBA,  L.    " Considerazioni   sulla  genesi  del  Piperno."  Giorn.  Min.  Pet.,  Ill,  1892, 

PP-  23-53- 

12.  DE  LORENZO  and  RIVA.     "II  cratere  del  Vivara  nelle  Isole  Flegree."    Mem.  Ace.  Napoli,. 

X,  1900,  No.  8,  pp.  1-60. 

13.  DE  LORENZO  and  RIVA.     "II  cratere  di  Astroni  nei  Campi  Flegrei."     Mem.  Ace.  Napoli, 

XI,  1902,  No.  8,  pp.  1-87. 

14.  DE  LORENZO,  G.     "I  crateri  di  Miseno. "     Mem.  Ace.  Napoli,  XIII,  1905,  pp.  1-25. 

15.  DE  STEFANI,  C.     "I  Vulcani  Spenti  dell'  Apennino  settentrionale."     Boll.  Soc.  Geol.  Ital., 

X,  1891,  pp.  499-555- 

16.  FANTAPPIE,  L.     "Sui  Proietti  vulcanici  trovati  nell'  Altipiano  tufaceo  occidentale  dei  Vul- 

sinii."     Mem.  Ace.  Line.  (5),  II,  1898,  pp.  547-575. 

17.  FUCHS,  C.  W.  C.     "Die  Laven  von  Vesuv."    Neu.  Jahrb.,  1866,  pp.  667-687;  1868,  pp. 

553-562 ;  i869>  PP-  42-59. 169-193. 

18.  FUCHS,  C.  W.  C.     "Die  Insel  Ischia."     Tsch.  min.  Mitth.,  1872,  pp.  196-239. 

19.  FTJCHS,  C.  W.  C.     "Monografia  geologica  dell'  Isola  d'  Ischia."     Mem.  Com.  Geol.  Ital.,. 

II,  1873,  pp.  1-58.     (Map.) 
192 


BIBLIOGRAPHY.  193 

20.  GUNTHER,  R.  T.     "The   Phlegraan   Fields."     Geog.   Jour.,  London,  1897,  pp.  412-435, 

477-499-     (Maps.) 

21.  HAUGHTON,  S.     "Report  on  the  Chemical,  Mineralogical,  and  Microscopical  Character  of 

the  Lavas  of  Vesuvius  from  1631  to  1868."  Trans.  Irish  Acad.,  XXVI,  1876,  pp.  49-164. 

22.  JOHNSTON-LA  vis,  H.  J.     "The  Geology  of  Monte  Somma  and  Vesuvius,  being  a  Study  of 

Vulcanology. "     Quart.  Jour.  Geol.  Soc.,  XL,  1884,  pp.  35-112. 

23.  JOHNSTON-LAVIS,  H.  J.     "  On  a  Remarkable  Sodalite  Trachyte  Lately  Discovered  in  Naples, 

Italy."     Geol.  Mag.,  Dec.  Ill,  VI,  1889,  pp.  74-77. 

24.  JOHNSTON-LAVIS,  H.  J.,  and  others.    The  South  Italian , Volcanoes.   Naples,  1891,  pp.  331. 

25.  KALKOWSKY,  E.   "Ueber  den  Piperno."   Zeits.d.  deutsch.  geol.  Ges.,XXX,  1878,  pp.  663-677. 

26.  KLEIN,  C.     "  Petrographische  Untersuchung  einer  Suite  von  Gesteine  aus  der  Umgebung  des 

Bolsener  Sees."     Sb.  Ak.  Wiss.,  Berlin,  1888,  pp.  91-121;    also  Neu.  Jahrb.,  B.  B. 
VI,  1889,  pp.  1-35. 

27.  MERCALLI,  G.     "Osservazioni  petrografico-geologiche  sui  Vulcani  Cimini."    Rend.  Inst. 

Lomb.,  XXII,  1889,  pp.  1-9. 

28.  MERCALLI,  G.     "  Contribuzione  allo  studio  geologico  dei  Vulcani  Viterbesi. "     Mem.  Ace. 

Line.,  XX,  1903,  pp.  5-38. 

29.  MODERNI,  P.     "Note  geologiche  sul  gruppo  vulcanico  di  Roccamonfina. "  Boll.  Com.  Geol. 

Ital.,  1887,  pp.  74-99.     (Map.) 

30.  MODERNI,  P.     "Le  bocche  eruttive  dei  Vulcani  Sabatini."  Boll.  Com.  Geol.  Ital.,  1896,  pp. 

57—112,  129—160.     (Map.) 

31.  MODERNI,   P.     Contribuzione  allo  studio  geologico  dei  Vulcani  Vulsini.     Roma,  1904,  pp. 

234.     (Map.) 

32.  RICCIARDI,  L.   "Ricerche  di  chimica  vulcanologica  sullerocce  dei  Vulcani  Vulsinii."   Gazz. 

Chim.  Ital.,  XVIII,  1888,  pp.  1-21. 

33.  RICCIARDI,  L.     "Ricerche  chimiche  sulle  rocce  vulcaniche  dei  dintorni  di  Viterbo. "  Atti 

Soc.  Ital.  Sci.  Milano,  XXVIII,  1885. 

34.  ROTH,  J.     Der  Vesuv  und  die  Umgebung  von  Neapel.     Berlin,  1857,  pp.  539. 

35.  SABATINI,  V.   "  Sulla  origine  del  felspato  nelle  leucititi  Laziali. "   Boll.  Soc.  Geol.  Ital.,  XV, 

1896,  pp.  70-76. 

36.  SABATINI,  V.     "Vulcano  Laziale."    Mem.  Carta  Geol.  Ital.,  X,  1900,  pp.  392.     (Map.) 

37.  SABATINI,  V.  "De  I'e'tat  actuel  des  recherches  sur  les  volcans  de  1 'Italic  centrale."     C.  r. 

VIII.  Cong.  G&>1.  Int.,  Paris,  1901,  I,  pp.  366-376. 

38.  SABATINI,  V.     "II  peperino  de'  Monti  Cimini."     Boll.  Com.  Geol.  Ital.,  1902,  pp.  1-12. 

39.  SABATINI,  V.     De  Petat  actuel  des  recherches  sur  les  volcans  de  1'Italie  centrale.    C.  r.  IX. 

Cong.  Geol.  Int.,  Vienne,  1904,  pp.  663-679.     (Map.) 

40.  SPECIALS,  S.     "Ricerche  chimiche  sulle  lave  degli  Ernici."     Boll.  Com.  Geol.  Ital.,  1879, 

pp.  301-302. 

41.  STRUEVER,  G.     "Studi  petrografici  sul  Lazio."     Mem.  Ace.  Line.  (3),  I,  1877,  pp.  1-15. 

42.  STRUEVER,  G.     "Contribuzione  alia  mineralogia  dei  Vulcani  Sabatini."    Mem.  Ace.  Line. 

(4),  I,  1885,  pp.  1-17. 

43.  TITTONI,  T.  "LaregionetrachiticadelP  AgroSabatino  e  Cerite."  Boll.  Soc.  Geol.  Ital.,  IV, 

1885,  pp.  337-376. 

44.  VERRI,  A.     "I  Vulcani  Cimini."     Atti  Ace.  Line.,  VIII,  1880,  pp.  3-34.     (Map.) 

45.  VERRI,  A.   " Osservazioni  geologiche  sui  Crateri  Vulsinii."    Boll.  Soc.  Geol.  Ital.,  VII,  1888, 

pp.  49-99. 

46.  VERRI,  A.     "Note  per  la  storia  del  Vulcano  Laziale."     Boll.  Soc.  Geol.  Ital.,  XII,  1893, 

pp.  39-80  and  XIII,  1894,     pp.  559-585. 

47.  VIOLA,  C.     "Osservazioni  geologiche  fatte  nella  Valle  del  Sacco  in  Provincia  di  Roma, 

e  studio  petrografico  di  alcune  rocce."   Boll.  Com.  Geol.  Ital.,  1896,  pp.  4-35.   (Map.) 


194  THE  ROMAN  COMAGMATIC  REGION. 

48.  VIOLA,  C.     "  Mineralogische  und  petrographische  Mittheilungen  aus  dem  Hernikerlande 

in  der  Provinz  Rom  (Italien)."    Neu.  Jahrb.,  1899,  I,  pp.  93-137. 

49.  VOM  RATH,  G.     "Das  Albaner-Gebirge."     Zeits.  d.  deutsch.  geol.  Ges.,  XVIII,  1866,  pp. 

510-561. 

50.  VOM  RATH,  G.     "Die  Gegend  von  Bracciano  und  Viterbo."    Zeits.  d.  deutsch.  geol.  Ges., 

XVIII,  1866,  pp.  561-585. 

51.  VOM  RATH,  G.     "Monte  di  Cuma,  Ischia,  Pianura."    Zeits.  d.  deutsch.  geol.  Ges.,  XVIII, 

1866,  pp.  607-639. 

52.  VOM  RATH,  G.     "Die  Umgebungen  des  Bolsener  Sees."    Zeits.  d.  deutsch.  geol.  Ges.,  XX, 

1868,  pp.  265-307. 

53.  VOM  RATH,  G.     "Zwei  Gesteine  der  Rocca  Monfina."    Zeits.  d.  deutsch.  geol.  Ges.,  XXV, 

l873»  PP-  243-247- 

54.  WASHINGTON,  H.  S.   "On  Some  Ischian  Trachytes."   Am.  Jour.  Sci.,  I,  1896,  pp.  375-385. 

55.  WASHINGTON,  H.  S.  "Italian  Petrological  Sketches:  I,  The  Bolsena  Region."    Jour.  Geol., 

IV,  1896,  pp.  541-566. 

56.  WASHINGTON,  H.  S.     "Italian  Petrological  Sketches:    II,  The  Viterbo  Region."     Jour. 

Geol.,  IV,  1896,  pp.  826-849. 

57.  WASHINGTON,  H.  S.    "Italian  Petrological  Sketches:  III,  The  Bracciano,  Cerveteri,  and 

Tolfa  Regions."    Jour.  Geol.,  V,  1897,  pp.  34-49. 

58.  WASHINGTON,  H.  S.     "Italian  Petrological   Sketches:   IV,  The  Rocca  Monfina  Region." 

Jour.  Geol.,  V,  1897,  pp.  241-256. 

59.  WASHINGTON,  H.  S.   "Italian  Petrological  Sketches:  V,  Summary  and  Conclusions."    Jour. 

Geol.,  V,  1897,  pp.  349-377- 

60.  WASHINGTON,  H.  S.     " Some  Analyses  of  Italian  Volcanic  Rocks."     Am.  Jour.  Sci.,  VIII, 

1899,  pp.  286-294. 

61.  WASHINGTON,  H.  S.   "Some  Analyses  of  Italian  Volcanic  Rocks,  II."    Am.  Jour.  Sci.,  IX, 

1900,  pp.  44-54- 

62.  WILLIAMS,  J.  F.   "Ueber  den  Monte  Amiata  in  Toscana  und  seine  Gesteine."   Neu.  Jahrb., 

B.  B.  V,  1887,  pp.  381-450. 

To  these  may  be  added  the  following  general  works  which  contain  descriptions 
of  some  types. 

63.  LACROIX,  A.    Les  enclaves  des  roches  volcaniques.    Macon,  1893. 

64.  ROSENBUSCH,  H.     Mikroskopische  Physiographic  der  massigen  Gesteine.     Stuttgart,  1896. 

65.  ROTH,  J.     Allgemeine  und  chemische  Geologic.    Berlin,  1883. 

66.  ZIRKEL,  F.    Lehrbuch  der  Petrographie.    Leipzig,  1894. 


INDEX. 


Absolute  chemical  characters 148 

Abundance  of  subrangs 172 

Acquapendente 4,  45,    49 

Age  of  eruptions 175 

Alban  Hills.    See  Latian  District. 
Albanose : 

Analysis  of 131,  135,  139,  147 

Description  of 130,  132,  133,  138,  140 

Occurrence  of 1S2,  137,  140,  161,  164 

Albanoso-jugo.se : 

Analysis  of 124,  147 

Description  of 123,  125 

Occurrence  of 125 

Amiatose 162,  170 

Analyses : 

Albanose 131,  135,  139,  147 

Albanose-jugose 124,  147 

Andesite 88 

Appianose 51,  147 

Augite 134 

Augite-trachyte 20,  147 

Auruncose 80,  83,  86,  147 

Beemerose 47,  147 

Biotite-latite 56,  88,  147 

Biotite-vulsinite 88,  147 

Braccianose 104,  109,  113,  116,  118,  147 

Cascadose  127 

Cecilite 139 

Ciminite 63,  75,  147 

Ciminose 59,  63,  67,  72,  147 

Ciminose-auruncose 80,  83,  147 

Giminose-monzonose  83,  147 

Fiasconose 127,  147 

Foyaite 51 

Gauteite 75 

Hanynitic  leucite-tephrite 51,  147 

Janeirose-appianose 51,  147 

Jugose 124,  147 

Latite 56,  88,  147 

Leucite-basal  t 135 

Leucite-basanite 127,  147 

Leucite-phouolite 47,  147 

Leucite-tephrite;.  .43, 51, 72, 80, 86, 92, 97, 101, 

104,  109,  113,  116,  117,  118,  147 

Leucite-tinguaite 51 

Leucite-trachyte 36,  43,  67,  83,  92,  147 

Leucitite 109,  113,  124,  131,  135,  139,  147 

Melilitic  leucitite 139,  147 

Minette 127 

Missourite 131 

Monzonose 75,  147 

Nephelite-syenite 47,    51 

Nordmarkose-phlegrose 20,  23,  147 

Obsidian 28,  147 

Ouachitite  . . .  127 


Analyses : 

Peperino 56,  147 

Phlegrose 20,  23,  28,  147 

Procenose-pulaskose 43,  147 

Pulaskose 43,  147 

Shoshonose 88,  147 

Table  of 147 

Tavolatite 51,  147 

Trachydolerite 75,  147 

Trachyte 20,  23,  28,  31,  36,  43,  59,  63,  75,  147 

Trachyte-obsidian 28,  147 

Vesuvose 122 

Vesu  vose-braccianose 118,  147 

Vicose 92,  97,  101,  147 

Vicose-ciminose 72,  147 

Vulsinite 81,  36,  147 

Vulsinose 31,  36,  43,  59,  75,  147 

Vulsinose-ciminose 59,  147 

Vulsinose-pulaskose 43,  147 

Andesite,  analysis  of 88 

Anguillara 6,  132 

Appianose : 

Analysis  of 51,  147 

Description  of 50,    54 

Occurrence  of 53,  164 

Appian  Way 51,    58 

Apulian  Region 1 

Arcioni 113,  114 

Arsal  monzonose : 

Analysis  of 75,  147 

Description  of 74,    78 

Arsal  vulsinose-ciminose : 

Analysis  of 59,  147 

Description  of 58,    62 

Arso 8,  74,  75,  76,  77,  78,  157 

Aspenal  monzonose,  analysis  of 75 

Astroni  Volcano 31,  33,  42,  43,  44,  75,  78,  157 

Atrial  braccianose : 

Analysis  of 116,  147 

Description  of 115,  117 

Atrio  del  Cavallo 8,  116 

Augite : 

Analysis  of 1S4 

Occurrence  of 157 

Augite-trachyte : 

Analysis  of 20,  147 

Description  of 19 

Auruncan  District.  .2,  7,  21,  26, 29,  33,  36,  40, 

61,  84,  88,  90,  92,  94,  110,  122,  164,  167 

Auruncose : 

Analysis  of 80,  83,  86,  147 

Description  of 79,  82,  84,  85,    87 

Occurrence  of 81,  84,  87,  161,  165 

Average  Magma 167,  174,  175,  179 


195 


196 


THE  ROMAN  COMAGMATIC  REGION. 


Bagnaia 41,  56,    57 

Bagnorea 67,  69,    70 

Bagnoreal  ciminose : 

Analysis  of 67,  147 

Description  of 67,    70 

Bagnoreal  vicose : 

Analysis  of 97,  147 

Description  of 67,    70 

Bagnoreal  vulsinose: 

Analysis  of 36,  147 

Description  of 40 

Barium,  distribution  of 188 

Beaveral  janeirose,  analysis  of 51 

Beemerose : 

Analysis  of 47,  147 

Description  of 46,    49 

Occurrence  of 49,  161,  163 

Berican  Hills 1 

Biotite-latite : 

Analysis  of 56,  88,  147 

Description  of 54,    88 

Biotite-vulsinite,  analysis  of 88,  147 

Bolsena 4,  81,  38,  59,  182,  161 

Bolsenal  vulsinose: 

Analysis  of 31,  59,  147 

Description  of 80,    84 

Bolsenal  vulsinose-pulaskose, analysis  of... 43,  147 

Boscoreale 107 

Boval  albanose : 

Analysis  of 139,  147 

Description  of 188,  140 

Bracciano 97,    99 

Braccianose : 

Analysis  of 104,  109,  113,  116,  118,  147 

Description  of.  103,  106,  107,  108,  111,  115,  117,  120 
Occurrence  of..  105,  107,  110,  114,  116,  120, 

161,  163,  164,  165 

Breccia .55 


Campanian  District.  .2,  8,  21,  26,  75,  78, 104, 

105,  116,  118,  120,  165,  166,  167 

Campiglia 1 

CapodiBove 139,  140 

Capodimonte 4 

Capranica 41 

Caprara 31 

Carbone 21 

Casa  Fredda 21 

Casi 26,    33 

Cavorcie  41,    65 

Ceccanal  fiasconose 129 

Ceccano 129 

Cecilite: 

Analysis  of 139,  147 

Description  of 138 

Cellal  venanzose 159 

Cerveteri 1 

Ciminian District....!,  5,  26,  29,  36,  39,  41, 
56,  57,  59,  61,  63,  65,  67,  70,  72,  73,  75, 

78,  81.  92,  94,  162,  166,  167 


Ciminite : 

Analysisof 63,  75,  147 

Description  of 62,    74 

Ciminose : 

Analysis  of 59,  63,  67,  72,  147 

Description  of 58,  62,  66,  67,  70,  71,    73 

Occurrence  of 61,  66,  70,  73,  161,  162,  163,  165 

Cimino  Volcano 5,  57,  61,  63,  65,  66,  162,  163 

Classification  of  rocks 11 

Clathrate  texture 109 

Colonetta 63,    65 

Comagmatic  region,  use  of  term —     T 

Copper,  presence  of 135,  149 

Correlation  of  types 141 

Cremate 78 

Croce  di  San  Marti  no 81 

Crocicchie 109,  110,  113 

Cuma 23,  24,    29 

Cumal  phlegrose : 

Analysis  of 23,  147 

Description  of 22,  27 

Dellenose 170 

Dikes,  rarity  of 144 

Epomeo  Volcano 8 

Euganean  Hills 1 

Fedorovite,  analysis  of 134 

Feldspar,  origin  of,  in  leuci tite 112 

Fiasconose : 

Analysis  of 127,  147 

Description  of 126,  129 

Occurrenceof 128,  161,  164 

Fiescolal  ciminose : 

Analysis  of 63,  147 

Description  of 62,    66 

Fiordinal  fiasconose : 

Analysis  of 127,  147 

Description  of 126,  129 

Fiordine 127,  128 

Flow  breccia 55 

Foglianal  ciminose-auruncose : 

Analysis  of 80,  147 

Description  of 79,    82 

Foglianal  vicose : 

Analysis  of 92,  147 

Description  of 91,    95 

Fontana  Fiescoli 63 

Formal  descriptions,  discussion  of 15 

Fosso  della  Parchetta 72,    73 

Foyaite,  analysis  of 51 

Frascati 182 

Frosinone 114,  185,  137 

Galeral  albanose-jugose : 

Analysisof 124,  147 

Description  of 123,  125 

Galeral  braccianose : 

Analysisof 109,  147 

Description  of 108,  111 

Galeroid  habit 109 

Gauteite,  analysis  of 75 

Glass,  occurrenceof 160 


INDEX. 


197 


Gradoli 45 

Grads,  use  of,  discussed 14 

Granatello 120 

Grignano 35,  86,  38,  89,    92 

Grotta  Ferrata 114,  132,  140 

Halite,  use  of,  in  the  norm 15 

Harzose : 

Analysis  of 56,  147 

Description  of 54,    58 

Occurrence  of 57,  162 

Hatty n it ic  leucite-tephrite 51 

Hernical  braccianose : 

Analysis  of 113,  147 

Description  of Ill,  115 

Hernican  District..  .2,  7,  112,  113,  114,  129, 

135,  137,  164,  167 

Highwood  Mountains 75,  127,  131 

Homologous  types 142 

Hornblende,  rarity  of 159 

Ischia 8,  19,  20,  21,  26,  27,  28,  29,  75,  78,  165 

Ischial  nordmarkose-pblegrose : 

Analysis  of 20,  147 

Description  of 19,    21 

Janeirose-appianose : 

Analysis  of 51,  147 

Description  of 50,    54 

Jugose : 

Analysis  of 124,  147 

Description  of 123,  125 

Occurrence  of 125,  161,  165 

Lagosello 94,  122 

LakeBolsena 3,  99,  102,  110,  122,  131,  132,  161 

Lake  Bracciano  . .  .6,  47,  49,  94, 109, 110, 132, 137, 163 

Lake  Mezzano 4 

Lake  Vico 5,  70,  75,    78 

L'Arso 8,74,75,76,77,    78 

La  Scala 118,120 

Latera  Volcano 4,  45,  49,  161 

Latian  District..  .2,  6,  51,  53,  110,  112,  113, 

114,  132,  139,  140,  164,  167 
Latite : 

Analysis  of 56,  88,  147 

Description  of 54,    88 

Leucite,  formation  of 181 

Leucite-basanite : 

Analysis  of 127,  147 

Description  of 126 

Leucite-phonolite : 

Analysis  of 47,  83,  147 

Description  of 46,    49 

Use  of  term 13 

Leucite-tephrite : 
Analysis  of.. 43,  51,  72,  80,  87,  92,  97,  101, 

104,  107,  113,  116,  147 
Description  of... 50,  54,  71,  79,  86,  92,  96, 

100,  103,  115 

Leucite-tinguaite,  analysis  of 51 

Leucite-trachyte : 

Analysis  of 36,  43,  67,  83,  92,  147 

Description  of 34,  40,  41,  67,    82 


Loucitito : 

Analysis  of..  ..109,  113,  122,  124,  131,  185,  138,  147 
Description  of 108,  111,  121,  123,  130,  133 

Madonna  della  Qnercia 65 

Madonna  del  Riposo 97,    99 

Madonna  d'Oro 114,  140 

Magma,  average 167,  174,  175,  179 

Magmas,  distribution  of 166,  172 

Marecocco 19,  20,    21 

Martinal  vicose-ciminose : 

Analysis  of 72,  147 

Description  of 71,    73 

Melilitic  leucitite,  analysis  of 139,  147 

Methods  of  analysis 10 

Minor  constituents,  determination  of 18,    60 

Missoural  albanose 130 

Missourite,  analysis  of 131 

Mode,  relation  of  norm  to 154 

Monnnal  shoshonose: 

Analysis  of 88,  147 

Description  of 88,    90 

Montana 51,  75,  127,  131,  169,  177 

Monte  Amiata 1,  158,  162 

Monte  Bisenzo 45,  99,  102 

Monte  Calveglia 45 

Monte  Calvario 1,     6 

Monte  Catini 1 

Monte  Cavallo 101,  102 

Monte  Cavo 6,  132 

Monte  Ciliano 57,    65 

Monte  Cimino 5,24,57,63,    65 

Monte  diCuma 23,24,26,    29 

Monte  di  Procida 26,  27,    29 

Monteflascone 4,  114,  124,  125,  127,  128,  140 

Monte  Fogliano 5,  92,    94 

Monte  Jugo 124,  125 

Monte  Levo 49 

Monte  Nuovo 28,  24,    26 

Monte  Pallanzana 61 

Monte  Rado 131,  132,  137 

Monte  Raschio 121 

Monte  Rotaro 8,  21,  26,  27,  28,    29 

Monte  San  Antonio 36,    92 

Monte  Santa  Croce 7,  88,  90,  165 

Monte  San  Vito 1,     6 

Monte  Somma 8,  105,  107,  116,  144,  165 

Monte  Tabor 8,    26 

Monte  Valentino 61 

Monte  Venere 5,  6,  7,  69,  70,  163 

Monte  Vico 5,  23,  24,  25,  26,  36,  39,  41,  59,    61 

Monte  Vulture 1 

Mount  Vesuvius 8,  104,  105,  107,  116,  165 

Monzonose : 

Analysis  of 75,  147 

Description  of 74,    78 

Occurrence  of 78,  163,  165 

Necrolite 57 

Nepheli  te-sy en i te,  analysis  of 47,    51 

Nordmarkose-phlegrose : 

Analysis  of 20,  23,  147 

Description  of 19,  21,    22 

Occurrence  of 21,    26 


198 


THE  ROMAN  COMAGMATIC  REGION. 


Norm  and  mode,  relations  of 154 

Noselite,  disuse  of,  in  norm 15 

Obsidian 27 

Olibano 21 

Orchi 26,  40,  83,    84 

Orciatico    1 

Oriolo 49,  121,  123 

Orvietal  a  u  run  cose : 

Analysis  of 86,  147 

Description  of 85,    87 

Orvietal  vicose: 

Analysis  of 101,  147 

Description  of 100,  102 

Orvieto 4,  70,  87,  94,  101,  102,  125,  161 

Osteria  del  Tavolato 51,    53 

Ouachitite,  analysis  of 127 

Paglial  procenose-pulaskose : 

Analysis  of 43,  147 

Description  of 41,    46 

Pallanzanal  vulsinose,  description  of 40 

Panza 26 

Peperino 55,    57 

Petrisco 41 

Petrographic  province,  use  of  term v 

Phlegrean  Fields... 8,  21,  23,  24,  26,  29,  31, 

33,  42,  43,  75,  78,  165 
Phlegrose : 

Analysis  of 20,  23,  147 

Description  of 19,  22,    27 

Occurrence  of 21,  26,  161,  162,  163,  165 

Phonolitic  trachyte: 

Analysis  of 23,  147 

Description  of 22 

Pianura 26 

Pianural  phlegrose 26 

Piperno 26 

Pofi 7,  114,  135,  137 

Poggio  Cavaliere 74,  75,  76,    78 

Poggio  Cotognola 97,    99 

Poggio  Muratella 47,    49 

Poggio  Tondo 49 

Porano 102 

Portici 120 

Preta 122 

Proceno 42,  43,  44,    45 

Procenose-pulaskose : 

Analysis  of 43,  147 

Description  of 41,    46 

Occurrence  of 45,  161 

Procida 8 

Progression  of  types 169 

Pulaskose : 

Analysis  of 43,  147 

Description  of 41,    46 

Occurrence  of 45,  161 

Panto  di  Castiglione 26 

Quartz,  rarity  of 159 

Badicofani 88 

Bare  earths,  occurrence  of 42 

Begional  progression  of  types 169 

BoccadiPapa 113,  114 


Bocca  Monilua 7,    90 

Roccas  trada 1 

Bock  types,  table  of 14 

Bomal  albanose : 

Analysis  of 131,  147 

Description  of 27,    29 

Bomal  Vesuvose : 

Analysis  of 122 

Description  of 121 

Bonciglione 41 

Botaral  phlegrose : 

Analysis  of 28,  147 

Description  of 27,    29 

Botondella 43,    45 

Sabatinal  beemerose : 

Analysis  of 47,  147 

Description  of 46,    49 

Sabatinian  District....  5,  47,  49,  94,  97,  99, 

106,  110,  114,  121,  122,  132,  137,  140,  163,  167 

Saccal  albanose : 

Analysis  of 135,  147 

Description  of 133,  138 

Sacco  Eiver 7 

San  Francescov 129 

San  Giovanni  di  Bieda 61 

San  Lorenzo 132 

San  Martino 39,  72,    73 

SanBocco 36,    39 

Santa  Maria  di  Galera 110 

Santa  Trinita 94 

Sassi  Lanciati 122 

Scalal  vesuvose-braccianose : 

Analysis  of 118,  147 

Description  of 117,  120 

Scanella 21,    26 

Selva  del  Lamone 4 

Serial  chemical  characters 148,  150 

Shoshonose : 

Analysis  of 88,  147 

Description  of 88,    90 

Occurrence  of 90,  165 

Sodalite,  disuse  of,  in  the  norm 15 

Sommal  braccianose,  description  of 107 

Sorianal  harzose : 

Analysis  of 56,  147 

Description  of 54,    58 

Soriano 57,    65 

Succession  of  types 176 

Tavolatal  janeirose-appianose : 

Analysis  of 51,  147 

Description  of 50,    54 

Tavolatite : 

Analysis  of 51,  147 

Description  of 50 

Tavolato 51,    53 

Teanal  ciminose-auruncose : 

Analysis  of.... 83,  147 

Description  of 82,    85 

Teano 26,  83,    84 

Thenardite,  use  of,  in  the  norm 15 

Ticchiena 134,  135,  137 


INDEX. 


199 


Tolfa 1 

Torre  Annunziata 120 

Torre  del  Greco 118,  120 

Toscanella 86,    87 

Trachyandesite 57,    66 

Trachydolerite,  analysis  of 75 

Trachyte : 

Analysis  of 20,  23,  28,  75,  147 

Description  of 19,  22,  27,    74 

Trebianello 110 

Trevignano 132,  140 

Tuoro 84 

Tuscan  Region 1,  2,  3,  145,  158,  162,  170 

Tusculum 140 

Type,  definition  of 12 

Types : 

Correlation  of  141 

Descriptions  of 9,    12 

Progression  of 169 

Succession  of 176 

Table  of 14,  147 

Valentino 125 

Venanzite 159 

Venetian  Region 1 

Vesbal  braccianose : 

Analysis  of 104,  147 

Description  of 103,  106 

Vesbian  Volcano.    See  Vesuvius. 

Vesuvius 8,  104,  105,  107,  116,  118,  120,  159,  165 

Vesuvose : 

Analysis  of 122 

Description  of 121 

Error  in  regard  to  name 104,  123 

Occurrence  of 122,  161,  163,  164,  165 

Vesuvose-braccianose : 

Analysis  of 118,  147 

Description  of 117,  120 


Vetralla 59,    61 

Vicarello 49 

Vicoite 95 

Vicose : 

Analysis  of 92,  97,  101,  147 

Description  of 91,  95,  96,  99,  100,  102 

Occurrence  of 94,  99,  102,  161,  163,  165 

Vicose-ciminose : 

Analysis  of 72,  147 

Description  of 71,    73 

Vico  Volcano.... 5,  22,  23,  25,  26,  36,  39,  41, 

61,  73,  78,  81,  92,  94,  157,  162,  163 

Viterbal  vulsinose : 

Analysis  of 36,  147 

Description  of 34 

Viterbite 40 

Viterbo 24,  40,  56,  57,    73 

Viterboid  habit 35 

Vulsinian  District. ..  1,  3,  31,  33,  45,  61,  67, 
70,  86,  94,  99,  101, 102, 110, 114, 122, 124, 

125,  128,  131,  132,  137,  140,  161,  166,  167 

Vulsinite : 

Analysis  of 31,  59,  75,  147 

Description  of 30,  58,    74 

Vulsinose : 

Analysis  of 31,  36,  147 

Description  of 30,  34,    40 

Occurrence  of 33,  39,  41,  161,  162,  163,  165 

Vulsinose-ciminose : 

Analysis  of 59,  147 

Description  of 58,    62 

Occurrence  of 61 

Vulsinose-pulaskose,  analysis  of 43,  147 


Zale. 


21 


THE  UNIVERSITY  LIBRARY 

UNIVERSITY  OF  CALIFORNIA,  SANTA  CRUZ 

SCIENCE  LIBRARY 


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REC'D  JUL  2  3  1979 


30m-9,'72(Q4585s8) — 3A-1 


